psql? There's some code to detect commands that can't be run in a transaction block in src/bin/psql/common.c, maybe that's what you're looking for.
Or did you mean something else? How doesn't it "pick it up"?
I think he probably meant that he was getting a syntax error, even after making all the changes.
Grzegorz, I would have suggested to make an entry for VERBOSE in parser/keywords.c, but it already seems to contain an entry for VERBOSE. I hope you are using the "opt_verbose" rule in your gram.y in the CLUSTER [VERBOSE] case.