In the game, having the workforce and tools to perform the A check in an airport requires said airport to be a base, thus it's not possible to perform it where the a/c is not based.
There is obviously the case where you fly between 2 of your bases, but in the end it usually represents a minimal fraction of your operations.
However, some time ago, there was a discussion about the possibility of extra services, contract with maintenance companies, etc. None of it has been implemented yet, but this shows that it's a full reflection, and not an easy feature to implement. Moreover, having the A check sitting in between 2 legs of a route means reworking the way scheduling works.