IME, there are some exceptions to 12 hour rule, though pretty sure not domestically. For example, had ~14 hours between a UA arrival and SQ departure in SIN recently (separate tickets), and was able to have the bags checked through. It took a while, though the time between flights wasn't the issue.

also, even in some overnight instances bags will [by default] be checked through. Earlier this year, we had a ~10pm arrival into EWR and ~7:00am departure. Bags were tagged by default (self-printing kiosk at SFO) for final destination, though an agent, now-questions-asked and had no problems short-tagging to EWR on request since we needed them for overnight - she was even training a new agent on how to do this and explained why she was able to do it for us (overnight connection) and it want to be done in simple connecting situations.