To be "in the works" a fairly "regular" plaintiff would have to prove he'd gone to the U.S. Attorney for D.C., asked for quo warranto, had the QW declined, and THEN pursue the matter himself. (Since DOJ has now entered appearances in court cases to defend Obama's side, the DOJ has conflicted itself out of QW most likely. If a government QW is ever brought, a special prosecutor will be needed.)
A not-so-regular prospective plaintiff might be someone who has an "interested third party" interest (strong and "unique" individual injury) in the matter and could file for QW in his own right, from the very start.
People claiming wrongful job termination, provably instigated by a President whom they contend is without rightful claim to the office (ineligibility), would have that kind of "interested party" status to bring a QW. High-profile terminated people like this guy ...
http://www.latimes.com/news/nation-and- ... 0361.story
would look to be almost slam-dunkers in being found to have standing.