I talked with Alejandro offline and at a high level, we should be able to support cron-like scheduling using crontab syntax. We may have to do some refactoring, but it should be possible to modify the EL functions (e.g. current(-1)) to handle cron frequencies as we're typically resolving frequencies to specific datetimes and using those anyway. So, we'd have a new time unit called "CRON" and the user would specify the crontab syntax as the frequency. We'd have to create a function that can parse crontab syntax, and some object that can handle the calculations for cron stuff. The EL functions currently just directly use the frequency and do some math, but if we do some abstraction here, we can make a version that handles cron, handles days, a version that handles months, etc.
If we want to do this, or even something similar, we'll need to change the datatype of the frequency column in the database to a varchar instead of an int to be able to store the crontab in there; in anticipation of this, perhaps we should make this database change in Oozie 4.0 while we can, even if we don't have the cron stuff ready yet?