Thanks to everyone for your responses.

The snake had another bowel movement just after a shed last week, which required no enemas or laxatives. The lab results show no sign of parasites.

To answer the question re. "backing up": It's pretty clear that this isn't the case. The fecal matter doesn't push forward - it gets stuck in a hard mass almost a foot into her body, blocking the passing of the urates as well. Normally this snake passes something every 1-2 weeks, instead of every month or more.

I'm concerned that putting her off her food for a month might not be the best solution, given how long she's been between meals. Then again, she still seems strong.

Any suggestions would be appreciated.