It's possible to tag multiple artists formatted for MusicBee using the MusicBrainz Picard tagger. Currently Display Artist, Guest Artist and Remixer are supported. Performer requires a few more steps, but you can configure Picard not to mess with your Performer credits.
You'll need the latest versions of MusicBee and Picard: http://build.oxygene.sk/job/package-picard-win-daily/
Many thanks to Steven and to Picard developers, especially Sophist, for the updates that make this work.
Display Artist and Guest Artist Edit
This script will find all tracks with multiple artists (not including Performer credits) in the MusicBrainz database and set Artist and Display Artist tags appropriately, plus tag feat. artists as Guest Artists. (If you're using ID3v2.3 tags, Be sure that the "Join multiple ID3v2.3 tags with:" option is set to ";" rather than "/".)
$if($in(%artists%,;), $set(DISPLAY ARTIST,%artist%) $if($in(%artist%,feat.), $set(artist,$rreplace(%artist%,\\sfeat.*,)) $setmulti(GUEST ARTIST,$replace(%artists%,%artist%; ,)) , $copy(artist,artists) ) ,)
To use scripts in Picard, open the Options screen and go to Advanced > Scripting, then check the box at the top. Copy and paste the code. You can leave the formatting as shown here or put it all on one line, it doesn't matter.
Remixer often needs no special treatment. If a Remixer value is in the MusicBrainz database, Picard will tag it and MusicBee will read it into the Multiple Artist Splitter.
For cases where there is no Remixer set in the MusicBrainz database, this script will catch any of them in the form "mix by" or "remix by" and set Remixer for you:
$if($in(%title%,mix by ), $if($not(%remixer%), $set(remixer,$rreplace(%title%,.*mix by ,)) $set(remixer,$replace(%remixer%,\),)) ,) ,)
It's more difficult to identify cases where the format is "Artist mix", since it could also be a remix done by the original artist with a name like "Club mix." For now, you'll have to tag those manually.
Unfortunately, Picard doesn't have good handling for Performer tags in scripts. To ensure that Picard at least doesn't overwrite any Performers you have tagged for a track, go into Options > Tags in Picard and add PERFORMER and/or performer (may be case sensitive, based on file type) to the field that says "Preserve these tags from being cleared or overwritten with MusicBrainz data."
Picard tags Performer credits to IPLS (ID3v2.3) or TMCL (ID3v2.4), so if you configure the Musician Credit List Custom Tag in MusicBee, you will be able to access them and use them as you wish within MusicBee. With the Additional Tagging and Reporting Tools plugin and some creative regex, you can convert them into MusicBee's Performer tags. I haven't sorted it all out yet, but I will share the specifics here when I do. Sophist's Standardize Performers plugin for Picard will probably be helpful.
Picard Plugin Edit
Forum member vzell and MusicBrainz developer Sophist created a plugin that will tag multiple artists in Picard in some useful ways.
You can find it here (right click and save as musicbee_compatibility.py): https://raw.githubusercontent.com/Sophist-UK/Picard-Plugins/master/musicbee_compatibility.py