This seems to have been going around as “soft-seat” in Toronto for at least a couple of years before it’s attested as “soft-seater.” It’s weird that these all come from Toronto:
1987 Craig McInnis _Toronto Star_ (Oct. 16) “Fleetwood’s Mac is back from booze bout”: “I’m not so crazy about the ‘soft-seat’ places. But you can get people out of their soft seats.”
1987 Greg Quill _Toronto Star_ (July 15) “Brewery chases high-profile acts for Massey Hall”: to help produce high-profile rock shows in Toronto’s premier “soft-seat” venue.... aggressive competition for “soft-seat” acts