maybe such a system could also be visible to others than the GM? That way, a prospective player can quickly look at the list when (s)he's thinking "this looks interesting" and asking himself/herself "Who is in this RP anyways?"

Such a system would need the ability for GMs to remove players too. As sometimes players drop off the guild or get kicked from the RP. neither of which guarantees the quit RP option has been chosen.