One way you could do it with non PNP drives would be to have the following setup:
1) Create 1 library per disk array. Each library will have its own DDB (I'd go with global DDBs) all managed by your media agent (it usually isn't recommended to have more than 1 DDB per MA but this should work just fine as they'll never be used at the same time).
2) Setup a storage policy and have the primary copy pointing to the disk array that will always stay on site.
3) Create 5 new selective copies of your primary copy and assign them 1 per disk array. For each selective copy, select the montly full option. For the 1st copy, tell it that the month starts on the 1st, 2nd copy on the 8th, 3rd copy on the 15th, 4th copy on the 22nd and 5th copy on the 29th.
Now, what that will do is that the first Friday of the month, it'll try to aux copy the fulls to the 1st array, 2nd week to the 2nd array etc. The end result will be that you will have 1 on site copy and 4-5 offsite copies each having their assigned week's full backup. You might not always get to your 5th copy depending if you have a Friday between the 29th and the end of the month. Also, if you happen to mix up your arrays and don't put the right away online during the right week, the aux copy will fail as it will want a specific disk array, not any.
As for the retention on those copies, you could set it to 30 days, 1 cycle. This means that as soon as the new fulls are copied to it and a data aging is run WHILE the array is still connected to the network, it will purge the old copy. Set it to 2 cycles to keep 2 months per array if you want to play it safe since you might not always have your 5th copy.
This isn't perfect but I think it would work around the limitations of using a regular disk library and trying to achieve what you're trying to do. I'll keep trying to think about a better way to do it but so far, that's the only one that comes to mind.
Hope this helps!