Hi,
I tried some googling but nothing useful came up.

I would like to build a lightweight SSB voice keyer for battery
operated portable operations (VHF contests and the like).

MP3 players are getting cheaper every day, they're *small* and
*lightweight* and their batteries last long enough for a typical
contest.

Has anyone seen a design of an interface between MP3 players and HAM
transceivers? Modern RTX could easily use the VOX facility, but older
gear (like my Yaesu FT-290R) would need external PTT keying.

Any idea? Would a "standard" VOX circuit do the trick?

Thanks,
Paolo IK1ZYW