OK, I was able to figure out something for GFW. It wasn't actually that hard to find, considering that I'd never used Cheat Engine until yesterday.
Start the game, change the stage number to the relevant stage (A-1 is 1, A1-2 is 2, A1-3 is 3, A2-2 is 4, A2-3 is 5, B-1 is 6, C-1 is 11, Extra is 16), and press Esc-R to start on that stage. After that, it seems you can keep using Esc-R to restart without having to set the stage number again. Replays will crash (even if you set it to start on an x-1 stage), but it's for practice anyway.
Not sure how the ice area is calculated, it seems to be logarithmic somehow (is that how floats are stored?). Everything up to 1065353215 is 0%. 1065353216-1073741823 is 1%.
1080000000 is 3%, 1090000000 is 7%, 1100000000 is 18%, 1110000000 is 42%, 1120000000 is 96%,
1130000000 is 218%, 1140000000 is 486%, 1150000000 is 1116%, 1160000000 is 2627%, 1161527295 is 2999%.
Edit: Ice area is now set to be a float in the attached cheat table.
Power is based directly on ice area (so setting the ice area to 3000 or higher will give you max power). The displayed level won't change until your next level-up though.
A few other things are also included in the file from when I was testing. The difficulty level was because I was hoping setting it to Hard or Lunatic while in Extra would cause revenge bullets to appear. Unfortunately, it doesn't seem to have any effect. (The reverse, setting the difficulty to Extra while in the main game, gives the same effects as Lunatic as far as I can tell, except the starting spell bonuses are 100,000 higher and the medals are those of spell 5, 9, etc instead of 4, 8.)
Medals seem to be preserved when starting on a different stage, and I haven't found any side effects from using this for capture/gold medal purposes (as long as you don't play the main game on Extra difficulty or vice versa). Though I'm not sure if they should count if you set your power higher than something you've legitimately reached by that point in the game.