Hello,
Some hours ago I tried to respond to your request by posting a link to the Roland RD-300S svc manual, but somehow my post didn't show up. What am I doing wrong?. Anyway, now I have uploaded the svc manual to the files section. Although the RD-300S and RD-300SX may be a bit different electronically, the key sections are most likely identical. On page 6 in the svc manual it is described in detail how to manufacture special tools for the disassembly of the keys, Hope this can be useful. Good luck.
Bo