|Yikes! As kids get older, they start holding onto pee for longer periods of time, and when they let go, it can be forceful (as you've experienced!). I guess I'd make sure you have a liner in her diaper, but beyond that the only thing I can suggest is putting her in an Airflow/Rikki for naps (or right after).
My other thought, though, is that you could view this as a good thing, and if she's dependably peeing at that time, not only is she getting more routine and going longer periods (i.e. good things for PTing), but maybe you could try having her use the potty after naps since she's likely to meet with success. That's not launching full out PTing or anything, but it could be a good opportunity to start the dialogue, if that makes sense. It really depends on where you both are at (like if she's really negative right now, maybe it's not worth it), but it might be worth dabbling with a bit?