Funnily, last year when we held a similar tournament I actually included your proposal Adam for the Quarterfinals and beyond. I chose to cut because I think it's actually less fair.
I don't have any data for this, but I think it's known to most people who played on a high level that 1st player wins more often than the second player, and even if the rules were changed that all current ties would be awarded to the second player, first player would still be favourable. (Wins 1p>Wins 2p +Ties)
I think that would still be true if the both players adapted to this rule change. Now, playing an 8th game instead of awarding the second player a win takes even more of their chances away to win the match. So this rule tries to give the coin flip who starts the game the least amount of weight, given a limited timeframe. My personal favourite would be to add two games every time until one set doesn't end 1-1, but I don't think we can expect anyone to play that much.
Personally, I'm not too invested about this, I just would like the disadvantaged person to have the smallest disadvantage possible. And I think playing an 8th game slightly increases that disadvantage over the current rule.
To DVX: Regarding your other tiebreakers, I don't quite follow why those would be better tiebreakers. The only thing I can think of is to avoid confusion about the Dominion rules? We have roughly a 52-46-2 % situation on our hands (maybe even worse for the second player). Why no just give this small possibility of a tie to the player who's being disadvantaged in that very game instead of picking a metric that didn't have anything to do with the match?
This logic is a slippery slope, though. Dominion is an inherently unfair game because there is variance, along with the first player advantage. Why do you want to try and correct some of it by using data that you don't have (and probably doesn't exist) but you don't want to enforce identical starting hands?
It's not the place of the tournament organizer to attempt to balance out unfairness that they see in the game by changing the game rules, it's the place of the tournament organizer to get in the way of the game rules as little as possible in determining a winner. The whole "coin flip" argument you make doesn't hold because games of Dominion aren't coin flips, they are opportunities for a skilled player to beat a less skilled player, which is kind of the whole point of the tournament. Playing tiebreaker games is the best way to provide "fairness" in this capacity.
Even DXV agrees that the best tiebreaker in terms of "fairness" (which I think means having the more skilled player win more matches, in the context of this conversation) is my suggestion. It seems that if people are so concerned with their time and want to put a hard cap on the matches at seven games, the best solution is to have the tournament rules reflect what keeps the most in-game integrity intact and allow people to shorten matches (DXV has a list of suggested tiebreakers they could agree on!) if they want to.
Let's assume 256 entries to the tournament, so there will be 256 matches played. Let's be really conservative and assume that half of them will end 3-3 after six games. So we have 128 matches that go to a 7th game. If 2% of those games end in ties, we'll have about 2.5 extra games of Dominion played, throughout the course of the entire tournament, meaning that the average player will play 0.02 extra games of Dominion using my tiebreaker rule as opposed to something that caps the match at 7 games. The impact of this method is much lower than it originally appears, but you end up with a much better structure as a result.