I think people wanting to use the Replay Mod without owning Minecraft is a little strange.. and I don't know how plausible a web player is: that would require a ton more work (I'm guessing), because the original mod uses all of Minecraft's assets that would have to be recreated in a web version.
I think your best bet is to just Replay Mod videos uploaded to Youtube, there's already tons of Replay Mod content, and the mod hasn't even been out for a week! Just search something along the lines of "replay mod" in Youtube, many people credit the Mod in either the description or the title. There's even a forum section where people have been posting primarily Youtube videos.
I'm no expert, and I don't want to talk for any of the developers, but I don't think a web player is plausible. Not any time soon, at least.