Ex post facto laws are prohibited with respect to criminal laws but not necessarily in regard to Civil laws. Provided there is no Criminal penalty a law requiring repayment of services to the government would not necessarily be unconstitutional. For instance a tax law that is passed that raises taxes retroactively for earnings made before the end of the current tax year would not be unconstitutional even though the money was earned at a time when the money was subject to a lower tax.
There are a lot of laws that apply retroactively that manage to punish people and companies monetarily but not criminally that are passed every year.
I’d say they should pass it and then let Jill Stein either pay it or pay her attorneys or both.
Frankly the State could just sue her for filing a frivolous lawsuit and making a frivolous demand for a frivolous recount. So even if the law were ruled unconstitutional, she could still be sued for wasting taxpayer dollars on a lark.
And frivolous is being kind. Are they allowed to use the expression “temper tantrum” in a court opinion?
After reviewing this in more detail based on comments posted on this thread, my understanding is that the punitive aspect of a law may define whether it's truly considered an "ex-post facto law" under constitutional terms. And the best description I've seen along those lines is this: A law or order is not punitive if its scope is limited to the compensation of harm suffered.
I look forward to FR lawyers weighing in on this one.