I think I figured out a way to do this without a moderator, except for the initialising of the game:
At the beginning, the moderator shuffles all the decks and posts them in seperate numbered spoilers filled up with _ and in the Courier font, which has constant character length, like this:
Legends Deck:
1-Titan____________
2-Dragon___________
3-some other legend
...
Always keep track of which card is the top card of each deck. At the beginning that would be 1. Whenever you draw cards from a deck, start at that number and look at the corresponding spoilers, then announce which card is now on top (i.e. increase the number by the amount of cards you drew)
When bottom-decking a card, spoiler it in your post with the number after the current bottom card.
If all current top cards are announced in each post or tracked in the spreadsheet log, this should be not too confusing.