I didn't think of this before but, the solid wire forms a spring that holds the line on the button but if the stranded wire goes slack it can fall right off.
I know this happens, especially in F2D, but doesn't this constitute a splice? Splices are verboten, CL General 5.3.5.
If the actual flying line itself does not go around the button, eyelet, etc but stops short, and a different piece of wire it attached, how would that not be a SPLICE in the line? Granted, this is a discussion of semantics rather than functionality.