Most food based WAM items (cake batter, pudding, cool whip, etc) are NOT CONDOM SAFE as they contain fat, which is a no no for latex condoms.

Items that are condom safe should be solely water based and contain no fat or oil of any sort. The simpler, the better.

MSGunge from messysupplies.com is essentially food grade thickener, color, and water. J-Lube would be a good solution as it can be mixed in large quantities and can be acquired easily with a quick internet search. Another issue regarding condoms is keeping it on amongst all the gunk. You're essentially adding a LOT more lube than one normally would experience, so be aware of the possibility of it slipping off. I'd advise you to put it on as soon as you physically can, because messy hands and plastic packaging don't mix well.

If all else fails, and you're concerned about pregnancy/STI's, there are tons and tons of things that you can do to stimulate one another outside of penetrative sex. Messy 69ing can be an absolute blast, provided neither of you suffocates. Get creative and don't think about penetration being the be all end all of sexual experiences. Especially since your girl is considering indulging your kink in the first place. If you're both absolutely still hell bent on fucking one another, and insistent upon using condoms, use getting messy and a sexy shower afterward as foreplay, and have some crazy fun post shower sex after you're clean.