Option to erase tapes when all images have expired

or even an option for a secondary expiration - erase after X days of expiration.

It might be worthwhile to allow erase in background process - if drives are idle for X minutes, load a scratch tape and erase it...

Very good idea. 
At the moment you can use bplabel command to erase the tapes. This will free the data from the tapes and the VTL will report the tape as empty.
You can make a script to do the same. Something like this:

for all tapes that are in scratch pool and the VTL
do
bplabel -m $tape
done

it can be run from the cron or manually. It does not have the "x days" parameter, but it gives you what you want. 

This would be a good idea all around especially for "data sensitive" environments where even in a PTL it may be required to "purge" old data for security sake once a retention is complete. However, you'd obviously have to have the tapes available otherwise it'd cause a panic from the system trying to find them.