First of all, thank you for the interest!
According to JACS, vol. 74, pp.4262 (1952) (Post 464821 (https://www.thevespiary.org/talk/index.php?topic=6355.msg46482100#msg46482100)
(imp: "Selective Methylation of 5-hydroxyvanillin", Chemistry Discourse)) DMS is not selective enough for this purpose. SWIM's general impression is that DMS is much more reactive than CH3I, there is e.g. this procedure: https://www.thevespiary.org/rhodium/Rhodium/chemistry/345-tmb.html (https://www.thevespiary.org/rhodium/Rhodium/chemistry/345-tmb.html)
. Hest methylates the syringaldehyde by stirring in DMF at room temperature for 2 h in his dreams. This one definitely needs 8 h on the boiling water bath when performed with CH3I / DMF.
(Maybe dripping in the DMS slowly would raise the selectivity?)
p-tosic acid methyl ester or trimethyl phosphate would represent a somewhat more friendly choice of methylating agents. Dimethyl carbonate seems to need 150 C to work properly and this can often kill our aromatic aldehydes.
From the viewpoint of safety and availability the best reagent would be NaMeOSO3, by far. Can one alkylate, say, vanillin in acceptable yields with NaMeOSO3 in water as solvent, not in a solid phase with microwaves? Your microwave procedure with KEtOSO3 sounds very interesting, Antoncho, but I doubt good selectivity could be achieved under such violent conditions, and 5-OH-vanillin is supposed to be prone to oxidation.
Wraith