I'd suggest leaving the the 8va (etc.) where they are in smartshapes and then adding them to the custom line tool section.

As long as we're on this topic, the 8va/b and 15ma smart shapes need more improvements than OP requested.

1. Ability to edit opening and continuation text (not symbol), including using any font.

2. Ability to be non-horizontal.

3. Most critical of all, options for termination brackets. Two seem to be prevalent with publishing houses. The first is a termination that always results in a corner with a vertical termination bracket (irrespective of the line angle). See Ross p. 203 or Gould p. 30. The other style of termination seems to be continuing the dotted line all the way around, as in the example below. The key point is, irrespective of the angle of the line, the termination bracket is always vertical.

It is possible to achieve 1 & 2 with custom lines. But 3 is a challenge, and neither house style has a proper solution.

Finally, just for blue sky's sake, the ability to create an 15ma/8va/b line with arbitrary inflection points like this. (Though not necessarily with that termination, at least as an option.)

This example shows only horizontal and vertical lines, but there are also examples with sloped lines at inflection points, like this:

(Both of these examples happen to be Messaien, because I happened to think of his scores as easy to find examples of these kind of markings. Durand's house style seems to be the continued dotted line for termination, but my personal preference is a actually the one given in Ross/Gould. Unfortunately neither Gould nor Ross addresses the issue of non-horizontal lines.)

The ability to create arbitrary inflection points is a very good point (pun intended). Other smart lines, particularly piano pedal diagrams, would benefit from them greatly.