Apologies if there is an idea already on this topic, I was not able to find, and if one exists, please merge them.
In the attached image I have ballooned the end of a solid created by sweeping a profile along a spline.
One balloon works nicely, as it is connected to the end.
The other balloon does not work nicely, as I have tried to point it at a long edge. Only problem is it doesn't actually ever attach to the edge. You have to first place the balloon to the end, then move it to the long edge based on a spline, and then eyeball it. And if you want to make it look nice, you have to edit the arrowhead back to normal instead of the dot that is shown.
Please allow balloons to be attached to views of splines as you can with everything else.