I'm in the process of throwing together such a beast. Here's how I'm going to tackle the problem:

The devices we are talking about interfacing too are low data rate, 4800 and 9600bps (GPS and OBDII) and that's not even continuous. The empeg can handle much more than that.

The multiplexor can buffer the communications and send a whole 'line' at a time to the empeg and back again. Each line gets prefixed to indicate what device it's from/to.

Additionally, an option may be needed to enable/disable specific streams. (I don't know if OBDII needs long bursts of data yet.)

My plan is to build a board that supports two serial devices (GPS and OBDII) as well as providing an accelerometer for basic rally computer functionality (0-60 time, 1/4 mile, peak HP, that kind of thing.)