In the games this opcode is used, as far as I know, only for Nahal's Reckless Dweomer. There may be a couple of other uses, but no more than that. So I don't expect there to be wide knowledge about how it works, still... The Dweomer uses it with the option of choosing from Known Spells. And with this option the opcode works. I made a custom spell and made my illusionist learn it. When he cast it, he was able to choose from his range of standard spells. The problem is with the much more interesting option of choosing from a custom 2DA. I drew up one with a few homemade spells on it, and the character just won't cast them. The icon for the chosen spell flickers over the portrait, but nothing actually happens.
My first guess was that there was some kind of a problem with these spells, but the character can learn them separately, outside of the plug-in, and cast them. On the other hand, if I add to that 2DA a regular spell - I put in Magic Missile - then that spell is not cast either. What's going on?
Edit: as I was typing this, I thought of another possibility - that with the "Choose from a 2DA" option the character may need to have the spells on the list memorized. That would have been a bad limitation, because obviously the "Select spell" is a boot in the door and an entry to other spell options that a modder wouldn't want to give players directly. So I tested for it: made my gnome memorize a MM together with the 214 spell. And this time the magic missile did fly.
So there you have it - memorized spells only. Now, what could we do to bypass this limitation? Give the caster the spells on the list together with the casting, to expire in, say, 6 seconds? I'm open to suggestions.