hi mate, no i havent actually, the seats have gone on the backburner atm, still really interested in doing this, but also looking at a different, actually sporty car instead of turning the nav into something its not, i'm still keeping the nav but not sure i want to modify it anymore, so most likely i will be doing the seats next week
as for the rex seats, my mate who actually had a 98 wrx sold his car before i had a chance to pull one out and have a look, however, looking through autobarn the otherday i found some seats made by 'drift' they look just like the wrx seats just in black and with a racer 'drift' stitching on the back, better in the lower lumbar support then the nav and wrx seats(anyone else find the nav seats stick out too much in the lower back part instead of being more flat backed?) and for someone my size i seem to mold into them, once your in you cant move! the holes on the seatbase look like they would line up with the factory rails, but a spacer might be needed for the back two bolt holes, aswell as a longer bolt, and i'm not sure if that would be ok or not as far as legallities go, and as i said i havent actually taken a seat rail with me to have a look, i hope this helps!
btw, if anyone has done this, or any other seatswap using the factory seatrails, can they pipe up? if it can be done for minimal modification, then i'll be more interested in doing this and posting up a how to guide or something!