bbauer,
Off the top of my head, it seems like user-generated invoice numbers (as long as they are integers and unique), should be reasonably simple to implement using the approach you mentioned. You would have to check for uniqueness before inserting or updating, of course.
The errors you're experiencing in VS 2005 are very unusual. Can you send me a screen shot of the errors? Obviously it works for a lot of our customers who customize DNI, so it has to be a conflict with something on your machine. Email us at support@dotnetinvoice.com with a screen shot and let's track it down.
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Rob
DotNetInvoice Support