My personl site is hosted at hashbase.io (link) so that you can access it via dat and https. My canonical dat url is dat://pfrazee.hashbase.io.
Before I used hashbase I used Github Pages, and today it dawned on me that I can have dat://pfrazee.github.io work too. It's pretty simple if you know the dat dns spec.
All I had to do was add /.well-known/dat to my site (click to view). I put the raw dat URL of my site in that file, along with a TTL.
dat://40a7f6b6147ae695bcbcff432f684c7bb5291ea339c28c1755896cdeb80bd2f9/
TTL=3600
I'm still hosted at hashbase.io, but the dat site is "available" at the github.io domain now too!
Just got it working thanks to @millette's great advice!
Right away, Beaker Browser advertises "P2P version available" on https://specious.github.io and dat://specious.github.io works as a URL.
Big thanks to @cmrberry from GitHub support for helping me figure this out, as I was perplexed why it wasn't working when I only added the .well-known/dat file.