Filtering Flowspec Routes from/to BGP Peer

Hi List, i currently search some informations of what filtering is possible in Bird on Flowspec (flow4/flow6) routes. As i currently understand is, that the BGP Common Fields (Community, AS_Path, etc.) is accesible and filterable, but no flow specific attribute (src, dst, ...). As i understand it correctly, it wouldn't be possible to filter flowpsec routes on source/destination attributes. So in a normal BGP environment, i can filter flowspec per AS, but not granular as this AS are allowed to send flowspec routes with src/dst from a given prefix!? Is something planned to implement or is it there and i'm unable to find it? regards, tim