Hey, that's an interesting idea! I like it. You're right, the different length time limits makes one play some games faster even if you have the same time remaining in them. Your idea would allow an average to be made, and yet it would allow for emergencies if you made the cut to enter the tournament as it wouldn't be the norm.
Yes, keeping track and adding it to one's profile would make for lots of programming. Hmm, oh well. We're paying members, time to hit the suggestion box, eh? Improving our experience will certainly help the bottom line eventually and it will increase word of mouth and sales too. The more options a tournament director has should make for better tournaments. I think after awhile that certain combinations would shake out and the better ways to hold them for various time limits would prevail.