Catalyzes carboxymethyl transfer from carboxy-S-adenosyl-L-methionine (Cx-SAM) to 5-hydroxyuridine (ho5U) to form 5-carboxymethoxyuridine (cmo5U) at position 34 in tRNAs. 5-hydroxyuridine(34) in tRNA + carboxy-S-adenosyl-L-methionine = 5-carboxymethoxyuridine(34) in tRNA + H(+) + S-adenosyl-L-homocysteine Homotetramer. Belongs to the class I-like SAM-binding methyltransferase superfamily. CmoB family.