Roadracing World has the entry list and there are two Aprilia RSV4s entered under the JiR Junior Team Gabrielli banner.
One going to Max Biaggi's 21 year old Nephew Federico, and the other to part time 250 GP rider Federico Sandi.
Though Superstock does not generate the publicity of Superbike, the competion in that series is always close and almost always a better indication of the performance envelope possible from the machine that you will actually get to buy. Superstock regulations allow far less modification than Superbike, with the occasional exception of Ducati shoving an RS model into the series...
Aprilia won 4 races in Superstock from seasons 1999 to 2001 with Daniel Olivier of Spain riding the original Mille-R.