NOTATIO

I'm wondering how some of you handle an ad lib harp glissando.

generally, it appears that curved lines indicating the general idea of where this gliss. goes seems to be the most standard way of notating it.

I'm finding that no engraving program really lets you insert a curvy line that matches well with the lines used for a straight gliss. marking.

I've gotten the "go ahead" from a professional harpist to simply indicate with a single straight gliss, with "ad lib" added above, or to do a couple of shorter straight lines that indicate the general contour of what is required, adding the text "ad lib" in this case as well.

Any other insights?

I'd create a curvy line, but it turns out it simply doesn't fit properly every context of a long orchestral work with multiple incidences of ad lib glisses.

Besides, it appears impossible to get that sort of contour AND have it attach to both starting note and destination note.

I am not sure what shapes you are after, but I make the following technical diagrams in Finale by piecing together various curves that I create as note attached expressions. Unfortunately they don't stay locked together if the spacing changes so adjustments have to be made after the fact.
And one can make simple curves that are note-attached and thus do adjust to spacing changes by using the Dashed Curve Tool (Smart Line) with the spaces set to 0. And I guess would could make more complex shapes by using several and attaching the middle ones to notes that are made very small as to be almost invisible.

Have you tried using a multi-arced slur?

A multi-segment slur with the end thickness set to make the the slur equally thick throughout should give any shape you want. I am doing another Dorico trial and just checked it out.

it's doable in Dorico, and making the straight playback gliss line invisible by making it really small works as well.
but it still looks messy to my eye.