I do not know any tool which can do that. I even do not know if Maemo sscd/csd/... daemons or libsms/libcall/... libraries exports some API for it. Jonwil should know more if it is possible via some Maemo (dbus) API.

But I think it should be possible at least via sending ISI packets (direct communication with modem). Some documentation still exists, see: http://talk.maemo.org/showthread.php?t=94275