That doesn't exist at all and it would require a lot of work to recreate it, even if possible (and it is). The alternative is to generate such playlist on demand.
You can create pools using other playlists as source though. Not with the original random pools component, but check my JS version ( Playlist Tools) on my sig. Other special sources may also be used (LIBRARY, SEARCHBY, GROUP).


You would obviously have to create multiple pairs of 'source,num of tracks' if you want to create a playlist with more than one track. And change the queries for every desired artist. You have multiple examples on the menus (which can be edited) and also on files at presets folder.
I would not add 50 artists this way though, but you can easily add 5 tracks for 5 artists this way and then another 25 random tracks from library.

My source,5;My source,5;_LIBRARY_0,20
My source,ARTIST IS gone gone beyond;My source,ARTIST IS David Bowie;_LIBRARY_0,%RATING% GREATER 3
Also note the query may use dynamic evaluation too, i.e. using the selection as reference. So instead of ARTIST IS David Bowie, you can use ARTIST IS #ARTIST#. And whenever you run the pool, the #ARTIST# part is replaced with the current artist selected. Note it only works with a single track, don't expect it to expand to multiple tracks. This may be used for a 'Top tracks by Artist' pool, where you can generate playlist for any artist on the fly.
Don't expect any more guide from me though.
EDIT: I'm currently fixing an omission on my side, currently script only allows a single call to a playlist as source (due to not having the numeration like the other sources), not multiple ones like my example. Will be fixed today.