hmm, I'm not vested in your approach at all, but based on what you said originally, I'm not sure why the iRig would not work.
it's quite small
has a 1/4" bass input
has headphone out
you can just plug in and play, or you can load music files (from iPhone, iPad, laptop etc) to play along with it
iRig software runs on most small devices ... iPhone, iPad, Android tablet, laptop either Mac or Windows (not sure about Linux)
sound quality is very decent
easy set-up. once you set up the software, it takes just a minute to plug everything in and go
pretty affordable as far as these things go
just curious about why it would not work for you ?