A permanent feature would be much better - something like a checkbox to indicate the signal is active low and automatically changing the font of that pin (or label etc) to include the top bar.
If the quick answer is 'no, the software does not support this feature at this time', I'll understand. I was hoping I had just missed a setting somewhere or there was an easy trick to doing it.
TBH I'm not going to spend a lot of time learning how to modify SVG files, so perhaps the easiest workaround is to simply add an apostrophe (or prime marker) to the end of the text like this: OE' or R/W' as an alternative to indicating active low I learnt in my youth.
Thanks for the suggestions.