There probably aren't that many DVD player codes in there, and there's actually an automated process for cycling through them in search of one that works (although I haven't tried it).

Code libraries are a sticky mess in some ways. You have component manufacturers, then you have remote manufacturers, and between the two is no central authority or management of remote codes. Outlaw is using one of the better sources for code libraries with UEI (they're a big player in that market), but even that isn't a sure thing.

You might be able to turn something up at Remote Central - a Denon DVD-2200 code that will work for a Radio Shack or One For All remote (both of which are manufactured by UEI) or for many of the cable box remotes around (many of which use UEI remotes) would work for the 990's remote as well.