if the seat is mounted to a Car's factory sliders, it must be a homologated seat mount (I don't know if this even exists, a porsche 911 cup car, maybe?). An aftermarket slider that is NOT FIA approved is a grey area, but should be disallowed by the same logic I outlined above (earleir post). if the seat mount isn't marked, have some sort of proof that it is an FIA approved slider. sliders are not homologated with FIA seats on list 12 (8855-1999) and most likely not those to 8862-2009, which have specific mounts, possibly sliders, used during homologation and those part numbers being required for the homologation to be honored (by FIA). There is no FIA homologation for seat mounts themselves, and sliders are not said to be used in the common standard (8855-1999) so there's no "right" answer here.

A fixed mount has near zero requirements OTHER than it tie to the seat using the homologated bosses (side or bottom). THIS remains a potential problem, but if you and your scrutineer are comfortable with you sitting there, go ahead. if the mount is inadequate, it's your funeral.

you could argue all day about it, but there's a big hole in the rules about fixturing strength. stock sliders out, the assumption I feel is that aftermarket are to be FIA approved, but there's really no homoogation standard for that. so... yeah.