IIRC, Organic Syntheses has a prep of trimethoxybenzoic acid from gallic acid using dimethylsulfate.
edit:
Here is the link to the Organic Syntheses prep.
trimethoxybenzoic acid: http://www.orgsyn.org/orgsyn/prep.asp?prep=cv1p0537 (http://www.orgsyn.org/orgsyn/prep.asp?prep=cv1p0537)