Sorry to chime in so late in the conversation...
Here's my take on the discussion so far:
1. I'm thrilled to see that multiple instances are happening. Thanks, Miller!
2. If at all possible, we should go all the way and have fully independent instances. Anything else will likely come back to bite us, by requiring additional documentation and other support, confusing developers (and therefore giving rise to bugs), underutilizing multicore systems, excluding use cases that we haven't thought of yet, etc. (And I already have a few use cases where concurrent instances would have been handy.)
3. As I understand the current state, there are two problems that we need to solve to achieve independent instances. One is a revision of the API to introduce an additional instance parameter. That might be unpleasant to implement but seems feasible. (I always expected that we'd need a completely revised libpd 2.0 when multiple instances become available.) The other one is the shared symbol table.
4. Miller, you said in your original post that you ran into seemingly insoluble problems when trying to create per-instance symbol tables. Would you mind elaborating on this point?
5. Even if we're stuck with a single global symbol table, we may be able to make it thread-safe without having to resort to locks.
Cheers,
Peter