A 100 percent run would be complicated. Do-able, but complicated.
I went over the specifics in the other Wario Land II post, but basically, you would have to finish the game several times in one movie. There are several branches and several final bosses (and one final time attack level with the 'real' ending after all that).
One of the trickiest aspects, and the one that stopped me from working on my run, is getting the coins necessary in each level to do the mini games to get the treasure piece, and map piece. I found the quickest way to do that would be to causes a 'glowing' coin to spawn, which gives you 100 coins when you snag one. The problem I faced was forcing one to spawn, I just couldn't figure out the pattern to it, and couldn't get one to appear at will.
Since running around levels getting coins in every level would add a LOT of time onto the run, glowing coin spawning is really the most feasible way to get the cash necessary, but since I couldn't get them to spawn when I wanted to, I stopped the run. If anyone can figure this out, then that'll make the run very, very do-able.
All that you'd need to do then is figure out which order to do all the levels. You cannot get access to a level select until you finish the game once. Considering that going to the map screen and selecting a level takes time, it would be wise to do the longest route first, so you get as many levels as possible done without having a map screen inbetween them.
The other issue is the endings. Watching the credits each time you finish the game would get very, very repetitive. I believe the best way to avoid this would be to soft reset the game as soon as the data is saved (right before the credits sequence) as to avoid watching Wario run home every time. Then when you finish the FINAL level ( the extra time attack stage ) you'll see one final credits sequence, and the video should let THAT one run. On the other hand, the credits sequence varies a little each time, and since it's a 100 percent run, and not a quick-as-possible run, perhaps they could be left in anyway? Up to the discretion of the runner, I suppose.
Anyways, that's my bit of advice to anyone who takes up a 100 percent Wario Land II run. It's a good game. The first secret chapter, though the quickest, isn't the best set of stages to judge a run on.
