The way it works on the River services is a mess though. First you're stopped when entering a jetty by someone wanting to check your Oyster balance (who, in my last case, saw my photo ID and then concluded I worked for London Underground!), then before you board the boat you must tap in as they watch - or they demand you do it again!
I've been told that they'd prefer it if people got paper tickets or mobile tickets, as they clearly don't seem to like accepting Oyster.
Any system on HEX would need to be quick and easy, and with a way to cope with the inevitable situation where someone won't have enough credit. As such, I'd say it makes more sense to deal with the payment at the airport itself, not on the train.