Big problem I see with the idea of a jump in game in Advanced or NRPs is that without a character sheet, you can't really be sure if everyone's on the same page and the GM won't know if there's significant things about the character that needs to be addressed until after the game starts. Even in Advanced RPs, I usually get one or two character sheets that just aren't up to standard for various reasons, including the character being super overpowered, being a total Sue, the back story being inappropriate for the kind of game, the player didn't read anything about the OOC before running off to the skeleton, et cetera. In Free and to some degrees Casual it can work because standard are usually more lax and especially in the case of free, character sheets are already pretty much non-existent, so I've noticed that GMs tend to prefer dealing with poor fit players well after they've joined, which is a lot easier when you aren't doing a lot of world building and every character can have a huge impact on the story. 

I'm not saying it wouldn't be worth a shot, but don't be surprised if the post length, quality, and character standards are all over the place to a much higher degree than the other sections.