"Its not so much the start bit that I want to trigger the interupt, it just seems like a good idea."

Jeff,
good ideas may not be always feasible or possible...........since the start bit itself is not that important, why not use the serial port interrupt for the task? use the facility that is provided in the microcontroller...it will be much easier and you can turn to other modules of ur project faster...
best of luck,
let me know if this helped
regards
tushar