Quick post as it may help someone : I've changed my joiplay for the market one and since then the rpgm plugin became a hassle (which is weird I think the market joiplay is only one version behind the got one) and got a missing file error : system/Option.dt while it was on sd card : moving the game in internal memory did the trick but was a no go : this device is a bit old and doesn't have that much. BUT making a symlink to the sd card folder worked. Yeah I know, I hate android too. (I guess you could try bind mounting too). Also weird it's working from my NFS, while it's mounted by an ugly hack on my custom rom.
PS: Needless to say but joiplay HAVE the right permission to sdcard, some others rpg are working directly from it.