Yep the above cobble / soda to diamonds recipe works fine, just make sure to include the imports I’ve added, for reference I used the following:
import mods.MTUtils.Utils;
import mods.MTUtilsGT;
# CANNER
mods.MTUtilsGT.addCustomRecipe("gt.recipe.canner", true, 16, 6, [10000], [<minecraft:cobblestone>], <liquid:soda>*500, <liquid:water>*500, [<minecraft:diamond>]);
Which appears in NEI just fine.
Word of warning on using MTUtils, unlike regular Minetweaker recipes won’t be removed on reload and require you to do a full restart of the client/server, so if you start getting duplicate recipes close everything and reload.