Ok I have tried one on my own and i edited the check sum but had a small problem.
As you can see the checksum the phone is expecting is only 3 chars long, Ive tried putting a 0F and a F0 and no go. Im sure its just a hex noob problem on my behalf but alas its a problem.
14:49:37, September 09, 2010
ERROR: Flash failure: Interface AP-OS: Error verifying Code Group 42 checksums. File: 0x620F, Phone: 0x62F (Error Code: 31),
Detailed Error Details: Direction of the Error=No Direction, Command Value=4000000, Code Group Number=42
Device ID: 0
Edit: Thinking it may have been an error i made a cg42 file both ways, with your script and the tried and true way with the same checksum expectation.