My guess is the included source is going where it wants, and you have no say about it.
Furthermore, I'll take a stab that the source for individual commands are ordered the same,
no matter what order your program first calls them.

Imagine you can push routines such as SERIN, PULSIN, DTMF.. out of the first page or RAM bank
because you thought your code was more important.
Now all of a sudden, a bunch of PBP's timed routines have to do bank/page switching
and you find out it was you that had to accommodate those routines rather than the other way around.

One trick you can do, is borrow some system variables if you can figure out when they are not being used.
There are variables used only to transport your PBP vars to it's serial routine for example.
Any other time your code is not executing a "serious" instruction, those system variables are sitting around doing nothing.