Catalyzes the methylation of the lipid moiety of the intermediate compounds phthiotriol and glycosylated phenolphthiotriol dimycoserosates to form phthiocerol dimycocerosates (DIM A) and glycosylated phenolphthiocerol dimycocerosates (PGL). Belongs to the methyltransferase superfamily. Phthiotriol/phenolphthiotriol dimycocerosates methyltransferase family.