According to DiscordBee's presentation on GitHub, the plugin can use any standard / custom / virtual tag defined for the current track. In what follows, I suppose that you do not play your MIDI files directly with MusicBee, but have already converted them to .MP3 files (or similar) instead.
A virtual tag is computed from other tags, with functions such as substrings, concatenations, regular expressions, conditional values... It is evaluated each time it is displayed.
Whereas a custom tag is stored either in the music file itself or in the library. It is shown just as it is stored (no computation).
The question is: for each audio file, what can the soundfont name be inferred from?
You can write a virtual tag if :
* the soundfont name is shown in the audio track filename or in any of its tags (such as Comment)
* the soundfont name is surrounded by standard delimiters (e.g. square brackets)
Otherwise, you could add to your MP3 files a custom tag dedicated to the soundfont name, and populate it.