Is there such an animal or can I cobble one together?
It would be pretty easy to knock one together. You could start with this one,
http://tangentsoft.net/audio/cmoy/misc/cmoy-tangent-sch.pdf (http://tangentsoft.net/audio/cmoy/misc/cmoy-tangent-sch.pdf)
and add an input mixer based on inverting op-amp mixer stage.
You could convert the power supply section to one based on a 9V input and using a charge pump IC.