Andrew:
The Transactions tab on the Customer profile has an option to search by Document in which you select the document type from a drop-down list (IN, CN, DN CA etc). How about using the same technique but defaulting it to "IN" since I'm sure that is by far the most common search.

Well, that would add another field to select, and I think you want the look up as fast as possible. I'd rather that if you type a document number and press down arrow, select the third item on the list, that that specific customer is loaded.

I know that this will be an R&D headache, but we'll take a look.