Betaine HCL has a molecular weight of 153.61 g/mol, betaine anhydrous has a molecular weight of 117.148 g/mol. The difference is one hydrogen atom with a molar mass of roughly one and the bound chloride with the mass of 35.453 g/mol. The water in the stomach causes a protolysis of the hydrogen from the hydrochloride version of betaine, forming hydrochloric acid and betaine. Theoretically, 76.26% of the Betaine HCL should be available for absorption as betaine anhydrous (trimethylglycine).
Am I missing something? I don't see any reason why the remaining betaine shouldn't be equally absorbed (76.26% that is) as oral betaine anhydrous is. The carboxylate shouldn't react with the stomach acid any further. Neither should the methyl groups react with water or the hydrochloric acid.