It's clear now. I was under the impression that you could actually affect the destination field through some kind of tweak/plugboard (other than the "Use series as category" setting)
I don't see myself running calibre from source all the time since every release will overwrite any personal modification of the driver. But just in case, setting the series as the albums could be accomplished by modifiying?
Code:
db_added.Album = metadata_x.series
.....
lb_added.Album = metadata_x.series
(around line 2730 of apple/driver.py in function _update_iTunes_metadata)