Union slush funds would not be able to legally reimburse them for this. It could be a way to send some of them to jail if they try.
By definition, a "slush fund" has questionable origins and/or questionable expenditures. But it's generally "off the books" anyway.
$100/day could be managed by collecting cash from supporters, a few bucks at a time. $1,000/day is a lot harder, and $10,000/day is effectively impossible to hide.
If you read the bill, the legislator doesn't even have to pay it. All he has to do is refuse, and it's deducted from per-diem or travel expense reimbursements. The resolution prohibits it from being deducted from salary.
$100/day is "walking around money" to these guys. It's not enough to "encourage" them to do anything.
Sorry, did you just use “union” and “legally” in the same sentence? Come on now, be realistic.