I looked at the CHDK source (which I should have done yesterday I guess) and unfortunately those code pages are hard-wired, so changing my editor to allow 1253 will not help - CHDK itself will only accept those five values.
To make things work, someone will have to construct the 1253 table in lib/font/codepages.h (where there is a comment about it, but no data). Then 1253 could be added to the list of acceptable code pages in core/conf.c and things would (I think) be OK.
The trouble is, I have no idea how to construct the 1253 table and until somone who does volunteers to do the work, Greek will remain unsupported.
[edit] Actually, it does not look too hard. There's a Greek codepage table here
http://www.microsoft.com/globaldev/reference/sbcs/1253.mspxAlthough I can't promise anything, if you let me know your email address and camera type, I could try to produce a special build for you to test.