I think this question is being asked from a flawed premise. There's no particular increased risk of a transwoman being a sexual offender compared to other males - offending rates are pretty independent of gender identity.
The risk from self-ID being the principle for bathroom access is not "because trans people are all rapists" (they are really not) but that women can't challenge a dodgy-seeming strange bloke hanging around the ladies (who may not even identify as trans) because all instincts to recognise potential threats have been quashed. The same senses that can help us to know when we aren't safe are the ones which tell us a bloke is a bloke, and we all need to be educated out of such transphobia apparently. Thus said dodgy-looking bloke is given respect and sympathy because it is kindest to assume he's a real woman who hasn't managed to learn how to pass yet.
In the event that a trans person did commit an assault in the workplace, I think there would be an onus to prove that the self-ID policy was a significant contribution to the circumstances arising whereby the assault could take place.
In the majority of possible scenarios for a sexual assault, self ID would be irrelevant because a male-identifying man, or indeed a man who was accepted as one who chooses to dress and present in a feminine style without being officially recognised as female, could just as easily walk into a female changing room and commit an assault. If so the self-ID policy would not be to blame.
Obviously the crime would still be a crime, but the employer would not be especially more liable because of the self ID policy.
Now if the perpetrator had a history of sexual violence and the company had procedures in place that would have discovered this info of a non-trans-man but didn't discover it due to self ID and therefore nothing to safeguard female employees then they might be liable. But if the perpetrator has no prior convictions then no.
I think I would be less concerned using mixed-sex facilities in most workplaces (assuming cubicles are properly private) than in public. I think the self-preservation of not rocking the boat where you are employed must reduce the risk in workplaces, whatever the gender identity of the perpetrator. Possibly not in highly misogynistic workplaces.