I believe the plan is to release version 1.0 of the core spec before moving on to extensions.
In the Mozilla article I linked to above, did you have a look at the other elements to consider using section? Any of those of elements can styled as underlined and one or more of them is likely better suited for your use case than <u>. Although not all of these elements can be found in the primary CommonMark spec, some have been proposed as extensions already. Maybe the alternative mood/voice element which can be used for technical terms.