In seeking a working definition for the term radical pamphlet, I would suggest beginning with defiinitions of the two components of the phrase. Pamphlet may be the simpler of the two to define - according to a few dictionaries at hand, a pamphlet is generally a short printed work not bound into a cover, though it could be stitched or stapled. Pamphlets are typically single items or parts of a limited series, rather than ongoing publications such as magazines. They would usually have a single subject focus and/or advocate for a particular point of view, rather than being a creative expression of an individual "as an individual" (what we would now call a zine).
Radical, on the other hand, is a fairly slippery term. Derived from the Latin radix, or radicis, meaning root, in math and science contexts it referrs to the root or foundation - getting to the bottom of things, so to speak. In the social/political realm, which is the usage to which your question refers, it has taken on the meaning of "one who carries his theories or convictions to their furthest application; an extermist." (Funk & Wagnell's New International Dictionary) In a way this seems a contrast to the root meaning. The concept seems to be that in getting to the root of social/political issues, one comes to conclusions and courses of action that are outside the "mainstream" status quo.
As you work on your working definition and your subject areas, you might want to consider that there are "radicals" of many persuasions. The terms you list all seem to come from what (for lack of more precise terms) would be termed left politics. You will also find many pamphlets produced by radicals from the right. Consider for example the right-libertarian John Birch Society, a group that produced a great many pamphlets (see this collection inventory at PublicEye.org for a list of some titles).
The use of the term in our name, of course, does indicate an overall left orientation, though there is no unifying political ideology at work. We like to think that as librarians were are inclined to advocate research to get to the root of things, and that the name is based on that older definition of radical. (Plus it has a catchy ring to it.)
Another general definition of pamphlet
For what it's worth, the Online Dictionary for Library and Information Science emphasizes the word "polemical" in the history pamphlets:
"A nonserial publication consisting of at least 5 but no more than 48 pages exclusive of covers, stapled or sewn but not bound, usually enclosed in covers of the same paper as the text (or a slightly heavier grade). Pamphlets were first published in England to disseminate the polemical writings of 16th-century reformers but are now used mainly for material too ephemeral or too brief (500 to 10,000 words) to be printed in book form."
See http://lu.com/odlis/odlis_p.cfm#pamphlet
Early American Pamphlet Resources
I just began a pamphlet research project of my own last semester, and thought that perhaps a few items from my bibliography might be of help. My work was focused on Early American pamphlets in the era of Tom Paine and specifically Common Sense, but perhaps a few of the items below will help in your search? It seems as though the Howard Zinn piece might be closest to what you are aiming for, but unfortunately he didn't provide any citations for his article. I have been finding myself that there is little thorough work done about pamphlets in general, so please do share what you are able to find!
Adams, Thomas Randolph. The British Pamphlet Press and the American Controversy, 1764-1783. Worcester, Mass.: American Antiquarian Society, 1979. Print.
Bailyn, Bernard, and John B. Hench, eds. The Press & the American Revolution. Boston: Northeastern University Press, 1981. Print.
Bailyn, Bernard. Pamphlets of the American Revolution, 1750-1776. Cambridge: Belknap Press of Harvard University Press, 1965. Print.
Gimbel, Richard. Thomas Paine: A Bibliographical Check List of Common Sense, with an Account of its Publication. New Haven: Yale University Press, 1956. Print.
Hall, David D. Cultures of Print: Essays in the History of the Book. Amherst: University of Massachusetts Press, 1996. Print.
Kanellos, Nicolás. "José Alvarez De Toledo y Dubois and the Origins of Hispanic Publishing in the Early American Republic." Early American Literature 43.1 (2008): 83-100. Web.
Kucsma, Jason. Resist and Exist: Punk Zines and the Communication of Cultural and Political Resistance in America. 2000. Print.
Liell, Scott. 46 Pages: Thomas Paine, Common Sense, and the Turning Point to American Independence. Philadelphia: Running Press, 2003. Print.
Loughran, Trish. "Disseminating "Common Sense": Thomas Paine and the Problem of the Early National Bestseller." American Literature 78.1 (2006): 1-28. Web.
Newman, Richard S. "Liberation Technology: Black Printed Protest in the Age of Franklin." Early American Studies: An Interdisciplinary Journal 8 (2010): 173-98. Literature Resources from Gale; Gale. Web.
Nicolás Kanellos. "José Alvarez De Toledo y Dubois and the Origins of Hispanic Publishing in the Early American Republic." Early American Literature 43.1 (2008): 83-100. Print.
Slauter, Eric. "Reading and Radicalization: Print, Politics, and the American Revolution." Early American Studies: An Interdisciplinary Journal 8.1 (2010): 5-40. Web.
Sweet, Timothy. "Native Americans and American Identities in the Early Republic." American Literary History 13.3 (2001): 592-602. Web.
Tuckness, Alex Scott. "Discourses of Resistance in the American Revolution." Journal of the History of Ideas 64.4 (2004): 547-63. Print.
Zinn, Howard. "Pamphleteering in America." Artists in Times of War. New York: Seven Stories Press, 2003. 93-108. Print.