When it comes to gaining, I had this vague idea:
Play Overlord as Crown, crowning Overlord as Crown, about 10 times.
The last Overlord is played as a Ferry'd Artisan (or Altar trashing Fortress), gaining Mandarin, topdecking all your Overlords.
The remaining Overlords that were played as Crown the first time are played as Bridge the second time, reducing the cost of Colony into Artisan/Altar range.
Before this, put the +1 card token from Teacher on Overlord, such that you draw the Overlords back up.
Do this again, playing Overlord as a gainer (Ironworks seems nice) instead of Bridge the second time.
Fiddle like a maniac with getting the details right. Finish this off with playing Vault, then a Royal Seal, buying first Herald, topdecking everything, then Mandarin, topdecking and getting topdecked by the Royal Seal(s).
But then it occured to me: I can just stockpile a bunch of coin tokens, buy Travelling Fair a bunch of times, buy all the cards left in the supply, with Herald being the last card, and topdeck all the things.
All that's left is discarding all the cards.
Playing a Vault will accomplish this, but that strands a Vault in play.
Another way of doing it is this: play Overlord-as-Crown, crowning Overlord-as-Crown, crowning Overlord-as-Crown, crowning Overlord first as a self-trasher, then a Mandarin-gainer, which topdecks the remaining three Overlords. The second play of Overlords 3 and 2 can be as something other than Crown—in particular, it can be Lurker to gain Overlord from the trash, followed by Vault. Note that Lurker is a Mandarin-gainer if there is a Mandarin in the trash, and Lurker (on an earlier turn) can put one there.
If you add Overlords you can add plays of actions costing up to $5, for example a Black Market or Storyteller which lets you play one or more potions, just in case you need to buy a potion-cost card in your buy phase.
The kingdom would then be Overlord, Crown, Mandarin, Herald, Lurker, Page, Peasant, Rats, Vault, Black Market, <a free slot>. Events/Landmarks: Bonfire, Travelling Fair.
Note that this works without using coin tokens: on the megaturn buy phase, you can have $(12*5+30*3+39*2+52*1+9*5) if you 2-pile Platinum and Gold and Crown 9xPlatinum, which is $324 total; Mandarin will put those back in an order you choose (and, hence, know). Since there's at most 22 piles to drain (4 Victory, 5 Treasure, Curse, Ruins, 11 Kingdom) and it takes at most $13 per pile ($2 for Travelling Fair, the most expensive pile is Colony at $11), then we have enough: 13*$22 = $286 < $324. All the gained cards are topdecked with Travelling Fair, and all the others are top-decked by overpaying for Herald; each card to be topdecked this way has provided us with $1 through Vault.
EDIT: in order to gain a Bat, you probably need to have Vampire in the kingdom, so that takes up the free slot. You also need to play the Vampire, which casts a Hex upon both their houses your opponents. If you steal all their cards first, the worst this can do is give them a Copper or a Curse. Each of these will happen at most once per Hex-shuffle, and we only need to play Vampire 10 times, so this won't 3-pile Platinum, Gold and Curse (or, less likely, Copper). In any case, if we 2-pile Copper and Curse first, then steal their decks, we lose $7 worth of payload, but had a slack of at least $38. So the solution still works.