Dubreq used to do a special version of their Stylophone that had an input. The great thing about it was that you could run an external signal through everything: filter, modulation, delay etc. The sound also distorted nicely if you turned it up all the way, and it could even be used as an ultra-primitive drum machine with the squarewave LFO. This version doesn't seem to be available right now.
I've been experimenting with their new CPM DS-2 drone synth for the last few days. It has an input as well, but sadly the signal only goes through delay and reverb (not the filters). The delay is very lofi and dark, the reverbs I'm not crazy about, although the plate ones can sound very eerie and metallic. You could easily just this only as an effect unit for external signals.