That makes the tagging more complex but your try looks good to me.
Not allowed unter german legislation when “overtaking=no” is signposted. Apparently we have the more simple traffic regulations … 
So do I ! Additionally we could use “mv” for motorized vehicles in general.
Why not, if one microcar drives very slow and the other one at max speed? The signposting does not prohibit that imho. And does the same not apply to the undersigned “agricultural=yes”?
Why not allow the established tagging of overtaking=no for the standard case and additionally offer the option of specifying the rule by “overtaking:mv:overtaken:dtv=no” for those who like a more specific tagging?
Given a proper documentation would not make the use of such tagging too complicated. And it would allow a correct tagging of the “agricultural=yes” problem → “overtaking:mv:overtaken:agricultural=yes”.