Well, since you can't alter the database, I don't think there is a good way to solve this issue.

Personally, I would opt for an attribute or for an additional table that lists all of the banks. Then compare the text field (in the case of the separate table listing the banks) to the banks table.

Other methods might involve a call out to fdic.gov on some sort of interface. That would also require a call out to NCUA for credit unions, imho. I don't think these are viable options in most cases because of the delay that could be caused.