Whoa, hold your horses
Lazyweb posts can be fun. Especially if you didn't intend to do a lazyweb post, but rather just wanted to rant
I did find that the M34's predecessors do have Linux drivers. Screw Siemens for not providing tech specs. Siemens, you suck. However, someone was nice enough to offer me his M105. Perhaps that'll work, provided the thing allows more than just one connection. (Side note: am I the only one who feels that anything which matches "M[0-9]*" sounds like a gun model?)
Someone else placed a comment on my blog, saying that the Siemens S450IP might do what I want.
And then a whole bunch of people told me through three media (mail, IRC, and blog comments), that ISDN DECT basestations usually allow one to assign different phones to different ISDN lines. Add an ISDN connection to the asterisk system, connect that to the DECT basesystem, and you can connect a specific handset to a specific asterisk extension.