OK, I'll add Similar Albums (and map it to the same field name). It will be a semicolon-separated list of artist/album pairs. For each item, how about the format: "Help!\The Beatles" or "Help! [The Beatles]" ?
Of those two, I'm a vote for [the brackets], but may not have considered all pros/cons, and would defer to any strong preference of yours/sergio's. Two other choices might be "The Beatles/help!" or "The Beatles\help!".
The following is a bit off-topic (or part of a broader topic), but…
...I bring up these two other choices especially because I noticed something interesting about how MC handles slashes, at least with respect to certain fields. That is, I am on my iPad right now, but from memory, take note of the following:
1. with respect to the Genre field...
2. dbpa ripped "Pop/Rock" for (only) one of the five file formats I tested as "Pop\Rock" (I.e. a reverse/)...
3. within MC Panes view, Genre pane...
4. Showed "pop/rock" for files with respect to four of the five file formats...
5. But showed "pop", and then a twisty that you could click to have it then show "rock" for files with respect to that one of the five file formats.
Thus, the reverse/ seems to be a very useful way to get MC to reflect hierarchical relationships that can be manipulated within a single Pane, a fairly sensible approach especially with respect to genres. I do not know if the same functionality applies by default for user-defined fields, or if such feature could be incorporated in future versions. I will assume yes.
So back to the question at hand. If the above is the case, I think I might change my vote to "the Beatles\Help!". That is, use the reverse/ not merely as a convention but actually as a way to build in a specific feature. Thus, if one were to say that an album's track is similar to several albums by the Beatles, the Beatles would show up first as the option in the pane and then a twisty could be clicked, where you would see the specific Beatles albums that were noted as being similar.
Of course, the above would be academic if there are not many similar albums populated in the field, which in practice may be the case. Still, if the functionality is there for user defined fields, and no one sees a downside to this convention, I think that would be my vote.
Having said all this, I am perfectly fine using the square brackets. I just thought the above was sort of interesting.
I will reply separately to the other points above.