What if gateways passed on part of their delegate pay to their customers as some kind of bonus, or interest? Would there be any economic incentive to do this? Would there be any economic incentive to not hire and/or fire gateways that do this? Would this be a good or bad or neutral thing for BTS holders?
Yes, that's the way to do it, if done at all.
Offer the gateway delegate pay on the condition they use it give discounts to users to make use of their new BitShares features, for a limited time, such as 1 year.
Should only be done for really big serious gateways such as bitstamp or coinbase. Once you got a couple of major gateways on board smaller sites will want to do it too for free.
The problem is ensuring the gateway keeps the delegate position. If there was a way to 'lock in' a vote for certain time period then deals like this could be made a lot more easily.