[SOLVED]
used my noggin. and lots of playing round:
using rip with auto merge and split (check auto merge)
so select edge then Y
then with rip pull
with snap place where want (line up)
snap to likely an edge you subdivided it with only vertices. line along them.
-==old:==-
edit: oh it would also need to merge with face it’s super imposed on (flat surface & axis)
just trying to reduce the combersome-ness of this.
this would work well in tandem with knife tool.
wouldn’t mind having a edge extrude only tool.
(kinda forgot we could)