ok, so in this case i suggest build your own serial procedure using internal timer. Maybe interrupts will be an intelligent way to monitor serial comm start bit if your device do something else while receiving data.

but as i told previously, there's maybe some trick to do with crystal choice.... let's say SERIN @ 300 baud @ 4MHZ... 400KHZ crystal can maybe acheive SERIN @ 30 BAUD with a DEFINE OSC 4

regards