Death and resurrection!
I installed RockBox (2012-01-19 version) on my HM-801, didn't like it that much (many functions not working) and when I used the uninstall method in the first post, it "died". What exactly made it die is not perfectly clear to me, but it could be that I unplugged the USB cable too early or that I installed the 2012-03-04 version of RockBox instead of the stock firmware.
After mourning over it's death a little I finally took courage and opened the HM-801 to do the electrical surgery called by some the "old and ugly" uninstall method: bridging pin 5 and 6 on the Samsung chip inside the DAP. I could see that the chip name wasn't exactly the same as in the 601/602, but I gambled on that it was going to be the same two pins ... and it worked: RESURRECTION !!!
Tronz was unable to help in my case, but by using the steps provided by Pudu in post #11 (quoted in post #66) and looking at the photo provided by makejoke in post #15, I brought my HM-801 back to life. Thanks to you guys!
Here is my extra info for those who might need to do the same operation:
1. Open the back cover: Remove the three screws on the side of the HM-801 where the Hold-button is (you don't need to remove the GanQi Bay cover). After the three screws are gone simply swing off the back like a door.
2. Removing the inner cover on the back: Now you see a big plastic back and in each corner there is a small hole in which there is a screw holding it. Unscrew these and lift the back up (using a flat screwdriver between the back and the sides will help).
3. The PCB with all components are now visible. Using a small flat screwdriver bridge the pins marked with red in the photo below when plugging the USB cable into the computer. After about two seconds you should hear Winodws say "ding-dong". Then release the screwdriver. This will make Windows recognize that there is something in the other end, but it won't prompt you with info about it. We are not done yet.

(You will have to open Device Manager in Windows to find the unit. However, in my case it turned out that it wasn't necessary to reinstall RockBox and thus I just went to the next step.)
4. Run the RK27Upgrade_Image software and follow the instructions provided by Pudu. Select the folder where you have the downloaded and unzipped firmware image and run it (as described by Pudu). Then you should see lost of strange characters running in the window and a pause ... and wait ... Don't do anything until a new window pops up showing that Windows has found a device called HM-801. Then and only then the resurrection process is over.
Resources:
Download link for the RK27Upgrade_Image software is here:
http://mycroft.su/en/cat/portativnye_pleery/headdirect_corp_hifiman_hm801/mycroft_tools_v2dcc5c.zip
Download link for the latest version of the original firmware is here:
http://down.head-direct.com/HM-801-v0.21.zip
Edited by LarsHP - 3/25/12 at 3:59am