I have a tip for u which is using in one of my tv in living room (Sony 32EX550 with 2222 box and HT [Sony DAV TZ 215 with external RCA R,L stereo inputs only] ) and succeeded in getting dd 5.1 to a great extent than your current set up I used earlier.
*My tv supports Dolby Digital and having a 3.5 mm stereo output jack
*Connect STB to tv via a HDMI cable
*Use a high quality 3.5mm stereo to R,L RCA cable to connect between tv and HT (Me using Belkin cable....350 Rs.)
*Set DD in STB so that Stereo out turns off from STB
*Set any of your favorite Pro logic mode in HT
*Now the 5.1 dd signal arrives at tv and down scaled to stereo and passes to HT.
*Me getting stunning dd 5.1 audio 80% similar to a 5.1 set up in optical in my other room