Normally it has to do with ice age views of sovereign immunity, but all in all that does lead down a slippery path to disparate treatment issues. I know there are financial institutions out there with similar issues, but normally education on a senior management level clears up those problems.
You say you're not a bank, but you want a bank policy in regards to this. The thing is, even the banks who do not educate themselves enough about lending on native lands, are educated enough to know that a stated policy of this nature will land them in trouble, so I doubt you'll find anything from a source you'd like to copy.
And just an FYI
We do not lend to natives, or on native land because of CRA. We are an extremely profitable financial institution, as well as extremely safe (read conservative sometimes LOL). We lend to these clients because it's profitable. In fact, it's often MORE profitable (and safe) to use some of the programs out there to lend on native land than it is to your standard 50 year old white guy. Again, it's all about perception and education