How to best attach EL wire to clothing mostly-permanantly?
The obvious solution seems like the best here. Sew it on by hand. This requires basically no sewing experience; if you can sew on a button you can do this.
Don't put the needle through the wire. Rather, put the needle through the fabric from behind, wrap it over the wire, and put it back through the fabric on the other side. This makes a loop of thread over the wire. Repeat several times in the same location until you have a sturdy loop around the wire. Tie off your thread. Repeat in another location.
Using a thicker thread will make the process faster, because you won't need as many loops in each spot. Suitable threads include: a double thickness of normal sewing thread, embroidery thread, crochet cotton, even twine if you find the appearance and texture acceptable. Remember that the knot on the inside will be thicker than the thread itself, so consider how the knot will feel when the garment is worn.
If you like, you can make the knot and the loose ends part of the design. Use a thicker yarn or string, and tie the knot on the outside. Leave the ends an inch or two, or as long as you want the fringe.
You will need to experiment to figure out the best spacing of the attachment points.