Interesting this should come up.. I was playing around on Hodgdon's load database and noticed that they had some .223 and .308 subsonic rounds on there. The rounds used Trailboss/Titegroup/Clays powder and all traveled under 1100 FPS.
(Also, you can use said database to get a general idea on what 9mm rounds would be subsonic. It looks like most stuff 125gr and greater are under 1100fps.)
Has anyone tried this? Obviously there would be cycling problems in a semi-auto, but I would be more concerned about either a squib round not leaving the barrel or too much pressure from the powder burning too quickly or a secondary burn from the powder not being packed. I don't really have a use for subsonic rounds, but it is interesting they are listed as safe rounds on the manufacturer's website.