I suggest using the shape functions in the OpenLilyLib library.
Or it sounds like a laissez vibrer slur maybe. You can use the \extendLV function that is somewhere on LSR. If not, I can post it.
[Sorry, I am in a rush...] Andrew Vaylor Trucks wrote on 24/03/2021 11:51 AM:
I really need to find a way to drop in an unterminated custom length slur that extends past a barline.
