Most of my experiance with PIC's for HW controle is wiht using them as IO trunks to bitwise IO devices. With a Device that is allready Serial The PIC may not help much. Although thay do have on bord DAC's .

The model we use is that tasks not requireing EPICs and VME can use a
PC-linux -> RS232 -> PIC -> ( HWIO, steping motors , penumatics , ADC/DAC Etc... )

We genraly write a simple set of functions in Asm or C for the PIC, and then use the serial line to down-load and runtime specal functions in the PIC memory from PERL or C.

We can then call all the functions in the PIC as though thay are sub's in the controle program. this included all the passing of var's and such. Taken to its extream ( which wee have done for a Z80 MCU) , the controle system downloads all the softwhere to the MCU each time it's initalized.
If your current C program is just using the bits in the parallel port as bit's and not trying to time a serial clock, the PIC or Z80 can offer many more lines of IO, and make the controleing computer less important ( Ie. I use a HP caculator to do debuging work on my controlers once there installed. )

Bill N. to spell, perchance convay my self; Or stay mute, a lurker on the side