Hey Sunflower,
Since Mecanique's bootloader code is in hex form, I don't see a way to insert an osccon command, but maybe I am missing something.
Since it doesn't seem important which bootloader they use, in this case, I would use Tiny Bootloader: http://www.etc.ugal.ro/cchiculita/so...bootloader.htm . You can add the changes you need into the assembly source, then compile.
If I understand your project, you currently have it running on a Basic Stamp, but you need to do it cheaper, so you are turning to PBP. You say you wanted each location to have a copy of PBP and MCSP. But did not want to put a crystal on it, because it cost too much. Something doesn't add up?
Basic Stamp II $49.00
PBP + MCSP + PIC18F4620 = $304.90
Difference = loads of crystals
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