all of the documents I have seen do not specify the material, they specify the exemtion code (which within them will dictate material and manufacture details). My hydro place told me the brass eagle 4 ouncers are excemt from test (due to size and DOT-E code). This also falls in line with this doc. http://hazmat.dot.gov/regs/notices/nprm/98_3684.pdf
Also, here is the part in particular (I screenshotted it).
This table seems to jive with the one Jaan posted.
As for the exemption code needed for the 2x2 rule.... From what I understand. the tanks need the code to have the material and method of construction documented. Thats the only way the govt can be sure the tanks are safe. I guess when they are manufactured according to the DOT-E specs and they are that small, the safety factor is large enough to warrant no hydro requirement.... That and the permanent expantion ratio is probably so small that it cant be reliably measured.
So the aluminum only doesn't apply accoridng to the documents I have found. And no one that has said aluminum only can provide documents to back up that info.