Scheduled reminders sent from inappropriate email address (multisite)

So the situation is that scheduled reminders are send from the domain email address - but they aren't domain sensitive - causing them to be sent out from the first domain & hence all with that domain's email address.

Our customer's first choice would be to be allowed to enter the source email address as a freetext field (we've had that sort of request on civimail before). If freetext isn't acceptable I guess we could render the same from address list as on civimail - we'd just have to not load a domain sensitive list as the reminders are not domain sensitive - which is kind of confusing - but we could comment it into the code