Hey Randall,
I reached out to ESRI support before going forward with that idea Luckily we figured out this issue.
I already had a property set in the portal system settings. For some reason it didn't occur to me to add additional properties in the same bracket.
This is what I was doing:
{"WebContextURL":[webcontextURL]}
{"disableSignup": "true"}
This wasn't working because, as soon as the first property was read, it wouldn't search for the next set of brackets! As soon as I realized what was happening, I felt silly.
What I needed was this:
{"WebContextURL": [webcontextURL], "disableSignup": "true"}
Once I did this, it worked without issue.
Hopefully leaving this explanation here might help someone else as well