I don't see why it shouldn't work. All RPs require a certain amount of trust in the GM. Unless the GM tells the players the exact stats of an NPC, the difficulty of a certain task, and so on, players have to trust that the GM is treating them fairly. It's just not something people usually talk about, but it's there.
As for powers and so on, why would a freeform superhero game be any different from any other game that could involve fights and conflict? When it comes to conflict resolution I don't see any difference between a character in a game saying "I draw my gun and shoot at XYZ" and "I fire my eye beams at XYZ". It may look different in a superhero game, but that is just CGI, nothing that changes the way conflict is resolved. The only difference might be that players might expect more combat and bigger fights in a superhero game, but for that a freeform game might actually be an advantage, because players can't fall back on simply announcing die rolls but have to actually describe things.
Now character creation might be a little tricky, but perhaps you could have a rudimentary "system" in place for that, even if the actual gameplay is freeform. Something like, say, characters get a certain number of skills or perks or somesuch, and if they want to have more the GM gets to write limitations or drawbacks into the character's background.