When it comes to MBG, do it right or don't do it... Sorry, but in this case I would tell the customer this is how it works and these the approved applications, either give us a direct public connection for server-gateway mode, or do the DMZ properly otherwise things are not going to work right. If they won't work within the necessary established standards, refuse to do the work... It might sound like a whiny kid the way I wrote it, but it will be worth it in the long run.
I make exceptions in some areas and fudge things, but not with the MBG when it's being used for trunking.