Ive seen cupra seats in a mk3 (from the Ibiza that shares the mk3 platform), 6n and 6n2 seats apparently fit.
Ive also seen plenty of stuff fitted that has used cut around mk3 subframes such as BMW E36 leather seats, Audi TT seats, Porsche Tombstones, and the recaros out of Rover 200 series... If you know someone who is good with an angle grinder and a welder, anything can be achieved.