If the game has no victory condition, I think it would be the best to define one by your own judgement rather than having high XP levels unavailable. So I think it would be the best to call yourself @@@@ or @@@@@ if you think you deserve it, and add a comment which explains that the game has no victory condition and why you think you should be considered a winner anyway.
I think a good general rule for games without a victory condition would be that you can call yourself a winner if you have reached deep enough that you have witnessed everything that the game has to offer (if no new monsters or items have been appearing in the last few levels, then you are a winner). I think this makes sense with MPRDB, but probably not with games where new elements are not introduced progressively, or games where the difficulty constantly increases and reaches impossible at some moment (I suppose Berserk! falls in one or both of these categories). You should use your own judgement then, I think.