You can let them know that nothing is possible for this right now. If it is more important to have those hints than MathQuill, you can turn off MathQuill, either on a course by course basis using the Course Config page, or globally with a setting in localOverrides.conf.
I'll share a thought about this. The answer blank width (without MathQuill) is meant to make sure the answer blank would be wide enough for reasonable student submissions across all random versions. It was not intended to actually hint at the length that is needed, just to try to make sure the student doesn't need to side scroll within the input field. Occasionally, there are questions where for some random versions the answer is short, while for others the answer is long. It can happen because of how a generic answer might simplify in some cases. So if one really wanted the answer blank width to hint at the length of the answer, that length would need to be computed dynamically for different random versions of a problem.
And one more thing about this. (Now I am going further into just sharing my personal philosophy.) Online homework ideally (imho) simulates pencil-and-paper assignments. If math exercises were assigned from a book using pencil and paper, there would be no hints like this about how long the answer is when written.
I do know the feeling of having invested lots of time carefully thinking about how to set these widths, and then with MathQuill that was all for nothing. Then again, it's not entirely for nothing as these widths are still respected in PDF hardcopy output.