If a promotion opportunity becomes available at my current employer whilst I am maternity leave, is there anything to stop me from applying even though I would not be able to commence starting until after my maternity leave?
Whilst you are on maternity leave, you not only have the right to apply for promotion opportunities, but you should be notified of these whilst you are on maternity leave. I am assuming that since you are aware of this opportunity, your company did take the trouble of telling you about it, but it is possible that you just heard of it through a colleague.
If it is the latter, then you should raise the fact that you have not been told properly. If you have been told properly then when applying I suggest that you indicate that you hope that it will not jeopardise your application that you are currently on maternity leave and could not take up the possibility until after you return. It would take a brave employer to turn you down outright with this in the background!