I was looking at the forums and came up to the topic "Cube Drafting" from HarlequinCasts,
viewtopic.php?f=56&t=14948&p=156905#p156905where I realized that we may already have a way of supporting custom sets and as a result cube drafting, through the sets & rarity mechanism that we are using.
Say that one adds a set in Manalink.csv. More specifically, one would need to add (let's assume at the end) a new set in Manalink.csv, and sets the rarity of the cards he wants to use, so as to appear that they belong (also) into the new set. Be careful that one would need to comply with the rules of having a set in draft
to enforce rarity=(11+ Commons, 3+ Uncommons, 1+ Rares at least per set) or for singleton if one drafts the same set for all 3 packs, make sure the set is at least 360 cards (or 240 for 2 packs, 120 for 1 pack).
Next add the new set in Expansions.h, menus.txt and finally into draft.c
In the menus.txt the new set Name needs to be added at the section "@EXPANSIONNAMES" last and also update the number of available sets by one.
Adding to draft.c would be a bit more tricky, because the new set (Name, Number) needs to be added in a few places.
But hey!!! Gargaroz or Korath have updated the code and automated this by reading the menus.txt

(just found out myself

) so no touching draft.c !!!
Next one needs to update the .dat (for sure Rarity, not sure about the others) files through the Magic_Updater ???
And should work. Haven't tested but probably worth a try...