So, FWIW, here's the solution I came up with:
1) I've added custom properties "BackupRunning" and "BackupStartup" to all nodes, and made them required fields.
2) I set the nightly backup jobs to backup startup configs from all nodes with "BackupStartup == True".
3) Shortly after, I have a nightly backup job that will backup running configs from all nodes with "BackupRunning == True".
4) I plan on making a report (and hopefully this is possible) that reports them in sections:
a) Last running config backup for all nodes with 'BackupRunning == True'
b) Last startup config backup for all nodes with 'BackupStartup == True'
c) A list of devices that are set to not backup either.
d) A set of "Inconsistent Configuration", where one or both of those custom properties is set for 'true', but the node is not in NCM.
5) If #4 works, I may try to tweak it so that 4a and 4b only report on those devices whose backups are >= 24 hours old.
Anybody see any flaws with my plan, or any recommendations for improvements?