The classic way to to that is to specify an interface or route metric,
which is a simple integer. As I recall, the interfaces file lets you
specify that, and while the man page does not say whether a higher or
lower number means preferable, on most systems lower means better so
probably it is that way here too.

I agree that rewriting the
configuration file and restarting WPA_supplicant is a good
approach.

Arnt