How about having her help you clean up the mess when it happens? If you make the process of cleaning up the mess take a long time, then maybe she will find it's less work to just go in the potty than to do all the clean-up.
You could have her take a bath or do some form of extra long clean-up of her body, followed by a pre-rinse of her clothes and underwear, followed by helping you do laundry to wash the clothes. Have her be involved in the process or at least present and helping as much as she is capable.
It will probably be inconvenient for you too, but maybe it will help get back on track if she sees how much work it is and how much it keeps her from playing...especially if she misses out on a specific play activity because you ran out of time. Something like, "Oh we don't have time to do X before dinner now because we had to spend all our time cleaning up the potty accident. Next time, you can try going poop in the potty and then we can have time to play instead of cleaning up."
You don't need to be disappointed or mad or unhappy about it. It's just matter of fact...oh, we had an accident....now we must clean it up.
Mine haven't had an issue with regression so I haven't tried that, but it's what I would do if my kids went back to wetting their pants on a regular basis.