[szhorvat]

We were considering making http://matlink.org/ compatible with Octave. The feedback I got on this was part of why this wasn't done. To make MATLink user friendly, it needs to come with compiled binaries, which would be linked against Mathematica's closed source MathLink library.

If it is the case that GPL doesn't forbid this, I'd love to hear about it.

[belorn]

Yes, you can do that for internal (ie private) use.

The only caveat would be if your job is at a university, and you plan to give copies to students. Distribution would be legally impossible.