I understand what you're asking for. I don't think it is "Normal" to have a provider model. The URL Rewriting module from Microsoft for IIS7 has a provider model, but that does not make it "normal". Even so, I understand
what you are asking for.
Today in IIRF, the best approach for you would be to use a RewriteMap, which gives you the ability to map from one value to another. You can learn about it in the documentation. Currently, the RewriteMap uses a text file as the source
for the mapping information. What you want is a database source.
There's a proposed addition to IIRF that would add a ODBC map type.
I haven;t done any work on that; nor have I committed to putting it into IIRF.