How can I tone down the heat in hot oriental mustard, without making it sweet/significantly altering flavor?

I came across this article.

It explains where mustard get its heat from:

Mustard seeds come from the mustard plant, a member of the cabbage family. They contain two sulphur compounds, myrosin and sinigrin, as well as an enzyme, myrosinase. When the seeds are broken and water is added, the enzyme breaks down the sulphur compounds. The result is the sharp tasting oil that gives mustard its pungency, and helps explain why the name mustard comes from the Latin words mustum (much) and ardens (burning).

In addition, it recommends adding some flour to tone down its heat.

In the case of prepared mustards, the reaction is toned down by using additives such as flour.

Make the mustard by replacing 1/2 the mustard with flour and adding some extra water to compensated for the thickening properties of the flour, then cook it off to get rid of the floury taste and there you go! If you don't want to go down the flour route you could always add coconut milk/cream or plain yoghurt instead of water which would temper the heat but you'd have to make sure that it suits the flavor for the appropriate recipe.

Hope the advice helps!

Replacing about 60 percent of the water with rice vinegar smooths it out a bit. It's still spicy heat, but not so harsh. When I started using the rice vinegar, mine started tasting a lot more like what we are served in Chinese restaurants.