IMHO $copy(_id3:xxxx,y) should be able to override Picard's internal tag mapping. ATM, it doesn't.
For example, Picard doesn't use TSOP or TSST fields with ID3 v2.3 tags. Some popular other programs use those fields, although they're not official. The user should decide for him/herself if he/she wants to use inofficial tag fields.
It is not possible to use for example $copy(_id3:TSST,discsubtitle) within tagger script though, it's being ignored.