Yes, just inherit from PredicateBase in the SDK. If you don't already have a custom extension, check out the Creating an Auto-Deploying Custom Extension Application tutorial to create one - it will also have a Sample Predicate within it.

Predicates in combination with variables (e.g. the Set Variable Value action, $BackupDB = true, then check for that later as necessary) are a good idea for the meantime. We've had requests that imported deployables should be allowed to be dependencies and that feature is scheduled for a future version.

We are planning to fix this in BuildMaster 4.2, but in the meantime you can edit it directly in SQL on the Plans or PlanActionGroups table (depending on if you're using shared/overridden plans). The format is a long, persisted XML string that hopefully shouldn't be too difficult to figure out.

Short of using the API, I can't think of a way to have the deployables be automatically removed based on variables. You could try a trigger, but that might not hit in time since those are on a few-second delay. The custom predicate sounds like it would work well.