How to deal with open questions

I’ve closed one, and posted in the other.

Ideally after a while a new topic is sometimes worthwhile – but I’ve answered in the thread, maybe that will generate a few answers by others.

Any ballpark suggestion on when it might be time for a new post on the issue?

I’d leave it for up to a week before posting again.

Or tips on how to politely goose responses our of old threads?

Bumping a topic within 24 hours on the busier forums is definitely discouraged and you may find that your bump posts are deleted on sight. Within the quieter forums (eg Hacks or Advanced), I’d suggest that you avoid bumping completely.

I’d suggest a new thread for a topic that’s 5 months old. Threads that have zero responses are far more likely to attract attentions from the forum regulars who use the No Replies filter. Added to which, there’s been at least one WP update since the first post, so the ballgame has (or should have) shifted from 2.9.x to 3. That tends to color responses too.

General rule of thumb: If you still have the same problem post-upgrade, it’s probably worth starting a new topic. Might be worth referencing the old threads to emphasis that it’s an ongoing issue, though. If it’s a core issue, the documented background might help to escalate it to Trac. If it’s non-core but a “nice-to-have” feature that’s currently missing, it may attract the attention of a plugin developer looking for a new project.

Does that help at all?