Beginning 1 September, we will need to geoblock Mississippi IPs

Might it be sufficient to dynamically block anyone that has a registered home address in Mississippi for their payment method? Most ISP's span multiple states.

Google have additional information about IP addresses that updates dynamically based on cell phone, wifi and other magic usage so maybe ask them if they have some javascript that queries their site for more specific city/state details. Also call Pornhub and ask how they were blocking specific states to meet legal requirements.

If I were in Dreamwidth's shoes, I'd be very much concerned with minimizing legal exposure, not number of users excluded. At 10k/user*day, it's a reasonable choice to block as broadly as makes sense.

Tough for the neighbors, but nitpicking "resident" is not a good choice here.