Apparently you cant. I just tried it.

Is there a way I can set the baud for HSERIN / HSEROUT conditionally based on an input pin?

I tried setting the registers manually like this:

RCSTA=$90
TXSTA=$24
SPBRG=8
SPBRGH=2

but then I get errors. I assume the HSERIN / HSEROUT commands require these to be set via the defines?