Never seen it done before but from what you've said then, yes you apparently can override an extension with a group. If it works for you then go ahead. Mailbox only extensions were originally intended to be used for group boxes so I think its fine, although I think it may confuse you in a year or two when you need to revisist it and figuire out what it's doing.

Good point.
But the problem is just keeping track of all the numbers, for example our office extensions are 5100-5299, I use it for home and some family and friends too, they are on 5300-5499 and I've started allowing some clients to use it on 6000+. I've yet to master clusters.
But on top of these extension numbers there greeting numbers, call group numbers and now group virtual voicemail numbers. Have set all greetings to be in the 4000-4999 bracket but call groups and virtual extensions are just all over the place. Have a system where the greetings broadly align with extension numbers ie. office is 5100 so first greeting is 4100, other office is 5200 so it's default greeting is 4200.
I'm sure to some my numbering system is just plain crazy, but everyone has their system I'm sure.
Cheers,
Adam