Thanks for the replies:
@chaley: thanks for the definitive answer: I figured it was not possible, but I just wanted to rule it out before I gave up on that solution.
@BetterRed: the reason is that I've created a composite column that is a mapping of author names to canonical author names (via a lookup table that's implemented as the "author link" column under "manage authors"). That part works fine: the new column holds all the author names, and they've all been canonicalized where necessary. But I then wanted to be able to view those (canonical) authors under the tag browser: however, if I retain "&" as a separator (which is what I was hoping I could do) then the individual names are lumped together as a simple string (eg. "bob & jane & clare" is treated as if it was a single author's name) and if I use "," as a separator then the names are split out individually as I want, but also any additional commas cause extra (bogus) splitting (eg. "Lloyd Biggle, Jr." becomes two authors: "Lloyd Biggle" and "Jr."). So I'm at a loss as to how to achieve the tag-like behaviour (canonical author names being split out in the tag browser) without having to eliminate the use of commas in my authors column (which seems like an ugly workaround). I was hoping that I could select something like "contains names" and have the composite column output treated as an ampersand-separated list, but that is apparently not possible. Any other solutions to this problem are welcome, naturally
@ilovejedd: thanks for the suggestion: it would be a good workaround, but the column does change fairly often: whenever I add a new book or article, or whenever I clean up the authors on existing books (which happens fairly often still because I didn't take a lot of care when I started adding books!).