I'm fairly sure that Bride rails (RO type) to suit the Ergo IIs & Brix IIs also suit Recaro S-series.
And bride rails for AE86s are a dime a dozen on yahoo japan, seriously. I wouldnt' bother mucking around adapting the SW20 rails - sell those to someone with an Sw20, buy some AE86 rails to suit the seat, and should be plug & play.