I can understand how you're thinking and don't know if i have a complete answer.
Phil has a good idea. If you partition the library (and always put the SQL tape in the same sub-partition, etc) and target the SQL job to that sub-partion, that should help your problem.
But, remember that media sets will have an impact, even then. The sub-partion tells backup where to physically find the tape. But, the media set defines the rules for whether or not that tape can be overwritten/appended. Try and keep that distinction clear and use the two properties to compliment each other.
I too think it has something to do with 'oldest available tape'.
Are you re-using the tapes in a cycle? - Mon SQL tape the following Mon (or 2 Mon later) for example?
If you are, try this. I'll assume you reuse them each week - 7 days later (adjust as appropriate). Set the overright protection on the media set to 6 days. On the job, be sure to tick the 'overwrite' option.
When you insert the tape, don't do the quick erase (overwite should take care of that) If you do an inventory, your media set should show that tape in blue - "available for overwrite". By doing the quick erease, you may be inadvertantly changing the "last accessed" date and defeating part of the reason for using media sets.
