Chris,
I feel your pain! As a large-ish consortium we've definitely needed to establish some standards around this, in particular where there are custom reports or elements depending on perf codes.
Fully extended, ours might look like this:
GTCS17BS01 = Granada Theatre Concert Series, Fiscal Year '17, Brian Setzer Orchestra, first performance.
or
RENT17AW03 = Rental, Fiscal Year '17, Ali Wong, third performance.
Your idea of using two digits for identifying the space would work - but I'm curious why that needs to be encoded in the Perf Code? We've developed our code format primarily to avoid ever having duplicate performance codes, so if you're often producing the same event in multiple spaces, I can see the need to distinguish between the event in the park versus the event in the studio!
Good luck with your standards-setting.
--Stacey