I'm curious here what happens after you mark a ticket as pending invoice? The accountant adds it into Sage and then properly closes the ticket? What happens if a technician (or client if you have the portal enabled) forgets to bill it and closes the ticket?
We could potentially look into ensuring exports respect the current filtering. I don't see much use in a notification when a ticket enters a custom state at the moment though.
There is the ability to mark a ticket as billable, which causes it to show up on the Unbilled Tickets By Client report however there's still no amazing way to show which specific unbilled ticket you're supposed to be looking for.
I don't think either of your solutions are great for the problem you have. That's not a slight on you, it's just that we don't really cater well for billing in a third party system (rightly or wrongly). I'm interested to know more details about this though. Having used Sage before I am not going anywhere near their API stuff, but we may be able to come up with something helpful in terms of automatically getting the useful data out.