I gave it a shot and was able to connect the control points and vertices of a spline to keypoints of other geometry:
If you hover over the spline, in a fashion similar to that for a circle to get its center point, the control vertices will show up and stay visible until you then mouse-over them to pick.
Are you also looking for something similar?
@David_McMahon, odd ....can you not place relationships on them either? ....also, are all the "IntelliSketch" glyphs selected?? [I have the "Relationship Colors" & "Alignment Indicator" buttons active] Check the "IntelliSketch Options" also.....maybe there are more unchecked choices there.
No I didn't spot your attachment, sorry......and yes, there seems to be something going on with your file.
Try my one. [attached - SE2019 file] ....see if you can add more relationships &/or dimensions.
Thanks for the post. I tried your file but could not get the constraints to work. I deleted the coincident constraint in your model and the snaps worked again. Not sure if it is a bug or something I don't understand with the constraint solver. Probably the latter.