I second the suggestion -- I've wanted this kind of feature for a long time. In my case, it's because the iPod displays only 3 fields when playing a track, and only one of them scrolls. That has implications for ways I might abbreviate field data to be more useful. I'd prefer to do that by creating custom fields for iPod transfer, rather than messing up the standard fields.
BVM, I've been doing a workaround to deal with this.
I have created cusom fields in MC, "ARTISTS" AND "ALBUMS". I copy the information in the ARTIST and ALBUM tag to the custom fields. Then I edit the ARTIST and ALBUM fields to give the abbreviated display that works best on my Zen (for example, on albums with multiple artists, I change the contents of the ARTIST tag to "Various," and shorten album names in the ALBUM tag where appropriate).
Thus, I've saved the original information, while the modified-for-Zen tags will automatically sync to the Zen.
In MC, I set my library view to display ALBUM ARTIST (or if I wanted to see all individual artists, to display ARTISTS) and ALBUMS, so I see the original and unmodified tag information when I'm in MC.
You can use this method to assign whatever information you want to whatever field you want on the iPod while still being able to view the original tagging scheme in MC. Of course, you have to remember what custom fields you saved the original tag info to - which is why I made it easy for myself by just adding an "S" to the original field name when creating the custom fields that I display in MC.
But I'd rather have the total flexibility that I suggested earlier so I keep the original info in the traditional fields.