Even granting that our hypothetical scientist is willing to take the risk of being admonished for working during shutdown

This doesn't seem to be that severe a grant. People go into science because they like it, not because it pays well.- for many, thoughts fields about one's subject can border on the intrusive. And as long as they come back and don't say explicitly that their new ideas were from when they were on leave, they'll be fine.

what exactly are they going to do without institutional support? No journal access, no computing resources, no facilities? Navel gazing only gets one so far.

So, they can read papers they already have. They can get journal access from friends at universities. They can do computing that doesn't involve as large a scale. They can think about data they got that doesn't seem to make sense. I agree there are limits but those limits seem not that restrictive as long as the shutdown doesn't last for that long.