One thing I meant to say, is if  \mathsf{abc} is implemented by 
switching to a sans font.
What to do about

$ U+1D5BA  U+1D5BB U+1D5BC $

that is


The choices (either justifiable) would be to let them go through to the 
sans math alphabet in the base font
or to give them mathcodes with ascii codepoints and the \fam of the sans 
serif font so that it typeset
as \mathsf{abc}

Unlike the switching in the other direction this would not need a 
mathcode switching loop in every instance
you could just set the mathcodes once, at the point the mathsf family 
was allocated.

Switching to use a text font is I think closest to the intended spirit 
of these characters and their use in  MathML.

<mi mathvariant="sans-serif">abc</mi>

is defined to be equivalent to


and distinct from


That is it's defined to be distinct although whether or not it is 
visually distinct naturally depends
on the fonts in use.

Thus MathML (at least) expects to be able to use runs of plane 1 
characters as multi-letter identifiers
and have them typeset as such, it doesn't mandate (or disallow) that the 
characters go straight through
to font slots with that index.