It looks like I'm making progress here... Thank you! As you suggested, it's important to leave the symbol "blank" for the custom barline not to appear at the beginning of a new staff (when the barline is at the beginning.
I also want to thank you once again for all your help and patience. Considering the music I'm writing and engraving, I'm sure I'll be writing again with a question! :-) I'd also like to be of service in developing or testing Denemo, so don't hesitate to let me know if there's anything I can do down the road.