Well in theory I don't see why not, it should certainly work I think, however unless you planned not to make full use of the obvious backup benefits, for instance Exchange and SQL Backup, it would be more expensive that way.
The way you propose you'd need a full copy of Backup Exec Server for the branch office, plus the remote agents, plus an Exchange agent (and SQL agent if you have Premium edition). However if the backup were done from the SBS server, you could use Backup Exec SBS edition, plus two remote agents, and backup everything on all three servers. Of course that depends on if you have a suitable backup drive at the main office, which may be why you're looking at this solution.
So yes, not necessarily how I'd like to do it, but certainly possible, though you do need to consider what you're going to do about Exchange, since you obviously can't have SBS at the branch office, you can't replicate Exchange there, and can't delegate a subdomain to it.
