This method works on old devices too. Today I tried it on Nuvi 350 and Nuvi 765T. It worked but folder name where ldr.bin and update.txt are placed must consist of for digits. Because HWID for these devices consists of three digits (476 and 870) we must add 0 at the begining of HWID. For HWID 476 folder name must be 0476.
For me doesn't work memory dump. When I tried to execute command rrgn (few versions) I got an error.
Parsing "rrgn,5,"
Error
Parsing "rrgn,16,"
Error
Parsing "rrgn,5"
Error: invalid command format
Command format: rrgn,<rgn #>,<output path>
Error
Parsing "rrgn,16"
Error: invalid command format
Command format: rrgn,<rgn #>,<output path>
Error
Parsing "rrgn"
Error: invalid command format
Command format: rrgn,<rgn #>,<output path>
Error
Parsing "reboot"
Success

Where is my fault? Why dump doesn't work? Some of these commands have invalid command format, but why other ones gave me an error?