So according to the math, even if you lose 100% of your game ones, you are odds-on to win the match if you win most of your postboard games (see http://www.channelfireball.com/articles ... in-legacy/).

Here's an idea then. Instead of building your deck with 75 cards, why not do so with, say, 200 cards. You still only have 15 sideboard slots so your main deck will contain 185 cards. With these 185 cards you use lots of cards of enough variety (including lands of all colours, for example) to adapt to any given deck. Then after game 1, which you will undoubtedly lose, you simply take out cards until your deck has a good matchup against whatever your opponent is playing. For example let's say a format contains a big three of decks, where deck A > deck B > deck C > deck A. You run deck "ABC" in your 200 slots. If your opponent is playing deck A, then after you lose game 1 you remove every card belonging to decks A & B from your deck, effectively "sideboarding" into deck C. Then you eat him for breakfast.

I've not seen this idea in event coverage and I only play Magic Duels. Does it work in practice? The rules seemingly don't care about how many cards you sideboard, so I don't see why not.

Uh, the rules do care about how many cards are in your sideboard. It must be legal at the start of every game, which is to say it cannot exceed 15 cards.

Yes, the idea is to have 185 maindeck cards and 15 sideboard cards, as usual. After sideboarding you have 60 cards maindeck, 140 in the sideboard.

but an 140 card sideboard isn't a legal configuration, so you can't start the game with your deck arranged like that. the rules don't require a 1-for-1 swap anymore but as far as I can find they didn't change it to allow you to go over 15 cards.

Another great idea bites the dust

Thanks for answers regardless!