I just creating a post with sort of the same question, but with a simple solution. Can't you just add a checkbox in the About panel with an option which "AUTOMATICALLY DEACTIVATE LICENSE WHEN QUITTING APPLICATION".
This means even if you forgot to deactivate the fixed seat, you can always check you subscription email and copy paste the license onto the machine you are currently running.
yes that would be a "fix". What would be better though is is the license was tied to me with the software checking online for the license and checking it out if a license was available. but I'm sure that would be more complex and your solution would be simpler since the software already has the ability to "deactivate" the license. Making the deactivation on exit an option would be a quick and clean solution.