Yep open source does not pay the bills at all and is probably one of the worst ideas ever conceived as I have earned diddly squat from any of my plugins even though a good 20,000+ are using one of the 4 I have developed.
However much I hate the idea of just giving code I have spent weeks developing away for free I have done so in case people find it useful. If you have a specific need for the plugin then you have the source code so code away!
At the moment you are the only person who has asked for this feature so it meets my definition of “custom” until someone else does or more importantly I need that feature myself as I initially wrote it to meet my own WordPress requirements not anyone else’s.
I have put the plugin up so other people could download it and change it if they need to and if you do that and want me to update the source I will put it into the next version if there is one and the code works.
So unless I suddenly find myself with lots of spare time on my hands or someone is going to pay me to do the custom work I doubt I will be looking into it anytime soon. Sorry.