Hello Henrik,
You don't say what Nav/Comm you are using, which may help the Dynon guys respond to your question more accurately. However,
I have a similar arrangement with a D100 as my EFIS and a D10A as my HSI using a Garmin 430 and HS34. All the flags (To, From, NAV, GPS, BC) appear on the D10A HSI screen and all the functions work fine, including VOR CDI, ILS LOC and ILS GS. You also get all the text info like TAS, NAV frequency, Bearing,Ground speed, winds and so on. I also really like the additional GPS indicator you can put up on the screen when you are in NAV mode. I have an RST marker beacon reciever, and it drives the marker beacon indicators just fine. I am not too sure how it would work with other recievers. When you say "HS34 link" are you referring to the DSAB connection between the Dynon units? If so, then I have quite a few hours on my plane with the HS34 installed, and I have had not problems with the version 4.0 firmware. So I find using the D10A to display the HSI and the D100 for the EFIS a really perfect solution, and you always have the D10A as a backup EFIS should you need it.
Best Regards,
Mel Jordan