Thanks for the quick answer. I did not get it to work. When I flash with the modified firmware using necflash (as root) I get:
Sending checksum 1 to drive
Sending checksum 2 to drive
Error while flashing firmware
Error submitting checksum
To make sure I did not make any mistakes hex-editing the firmware, I started with the original, made the 3 changes listed above to make it RPC-1, and then changed the checksum to what you said. The I made another firmware, this time starting with the fast+RPC-1 firmware, and undoing the 2 changes listed above, and then changing the checksum. Then I ran sha1sum on both files, and they were identical, so if I accidentaly have made a mistake, I must have made it twice, at the same place, which is unlikely.
I don't want to say that the checksum you gave me is wrong, since you know much more about this than I do, but I don't see what else could be the problem.
You wrote that the checksum is "calculated by a simple 16 bit addition". I understand that the checksum is a 16-bit value, and I know where it is, but I don't understand what I should add with what to get it.
I would appreciate it very much if you (or anyone else) could tell me the correct checksum, or tell me how to calculate it myself. Thanks.