Is it only supposed to work with Milk Drop?
As is now, yes. It's by design.
MilkDrop allows to be embedded, but I think the other default one gave problems in the past, so that's why I decided to play sure and only let it work on MilkDrop.
Anyway, I'd like to wait and see if visualisation embedding is in MM 3.0 or not. If not, then this tweak will be changed so that more visualisations are supported.
Why wait: because I think that embedded visualisation is a strange missing thing in MM (all other players/apps seem to have it).