can bug categories be associated with developers?
Yes. For example, I see that the "Regular Expressions" category is auto-assigned to effbot.
if so, would it perhaps make sense to add Peter as a developer,
That would be fine by me, provided he asks for it (it's a commitment, despite that developers routinely vanish for years at a time <wink>).
and add a category for the subprocess system?
That one's harder to call. Once a category is added, it can never be removed. So I'd wait on that to see whether enough reports came in against subprocess to justify it. Since it's unlikely that an original reporter will assign "the right" category to begin with, and the original submission is the only time at which auto-assignment has an effect, the major value in all this would be achieved simply by having a subprocess expert with commit privileges.