In scratchbox handling of stack overflows does not work. Therefore e.g. libsigsegv fails the autoconf checks during compile and a compiled version (e.g. stolen from Debian Lenny) won't work either.

Good news is that the current version of GNU Common Lisp in Debian Sid is already ported to armel. Still it won't build or run in scratchbox but you can use the Lenny build environment in qemu to build packets that run on the N900. libffcall is not required for gcl.

So what I've done is:
1. Installed a Lenny environment downloaded from here:
http://people.debian.org/~aurel32/qemu/armel/
2. Tweaked debian/control and debian/rules to remove dependencies on Lenny library versions. Installation prefix is changed to /opt.
3. Built packages for gcl and maxima.

The resulting packages work fine on the device, even plotting with gnuplot works.

Now I'm looking for a graphical UI. Most obvious choice would be kayali, since it's written in python-qt4, but that seems to be lacking lots of features and has been abandoned in 2007.