The Spammer Trick with the lower prio MX is a thing that should be hard to work around, especially when ure recieving Mail for several Domains.Armoring the Backup MX the same way like the Primary would mean syncing Spamfilter Data. In my case (ASSP) sure possible but a thing that should be installed for every aditional domain.File Access / Cronjobs would be needed on the Primary System, there should be FTP/SSH etc for File Transfer in Backup System
Mine "simple dirty fix idea" would be now to let the backup only recieve in case the primary mail exchanger is down.A little php/perl script [e.g. fsockopen()] could do this monitoring, writing postix conf /restarting for the case the primary dont respond as it should.
The advantage would be that the relay stays up.So spammers would get a responding tcp 25 presented on the Backup MX Record, maybe theres a possibility to delay them (lame them in fact...)
Im running some M$ Exchange servers , they are mostly in Client Workplaces on  thin dynamic-addressed DSL lines. This Environments are screaming for a BackupMX, so it would be a rly nice to have...
however, im not registered here.if someone got the tune to make such running, contact my bnc quakenet (plz / #dswp)