An expensive item you can buy from the vendor that increases the probability of forbidden materia for one use or until a duration passes if unused.
The gillionaires have something to spend their money on for better odds at the forbidden materia game... will probably want to price it accordingly if they try to sell it, and aw heck this isn't gonna work unless they fail a lot. And if they do it's not much of an investment anyway.
Unless: meld materia for others is implemented. Then have the use of this expensive item to mark the item as Untradeable. Rich get something for themselves, but can't get anymore rich off of it.
I'm debating with myself in a public forum, tis' embarrassing. I just wanted to throw it out there and see if it might be refined more, might be of interest, may not be of interest, scrapped, shoved in a moogle pit, etc.
edit1: Or for the gil sink, something that you buy from an NPC to even perform forbidden materia melding.