A nice program called Macindows is able to run some Windows-programs without Windows itself. I guess it's some sort of shell for Wine. Anyway, it seems to run Steem nicely (if a bit slow without auto frame skip). Just tried it with pasti.dll and couple of images and it runs surprisingly well

It's just a demo and expires after 30 days, but still it's a nice start. http://www.macupdate.com/info.php/id/33366/macindows
I also tried free program WineBottler ( http://mike.kronenberg.org/ ), but haven't got it working well enough with Steem. Maybe someone with more experience can help to build a working prefix for Steem?
Ofcourse the best thing would have Pasti-support for Hatari, but for now there at least seems to be one way to run STX on OSX.