You can alter the thumbnail text to show fonts the way you want them, using HMTL font properties. Here's an example (which I stole from a recent posting by marko):
<font color="a3a3a3">Your text goes here</font>
If these square brackets are already embedded inside your fields (like Name or Album), you would have to do some expression language magic to make this happen. My first blush at this is something like:
replace([Album],/[,<font color="a3a3a3">/[)
and
replace([Album],/],</font>/])
This is untested, but should give you the right idea. This will work for thumbnail text. If you are wanting to see this in the track details display, or Playing Now, I guess I'd make new calculated fields (like [Album Display]) and do the replacements above inside those new fields.
It's certainly not an easy answer, but if you really want it, I think it can probably be done.
Brian.