The ideas that come to mind are hosting via nginx or Python's
SimpleHTTPServer.
Both are going to require installing something (nginx is available in ports, the python module with pip).
Nginx will require a minimal configuration file (you should be able to find plenty of examples for serving a single static page).
Nginx is used to serve the FreeNAS web gui, so you could probably just start up another process without having to spin up a jail.
SimpleHTTPServer will require a jail.
SimpleHTTPServer just serves the current directory, so all it should require is putting together a script that 'cd's to the appropriate directory and starts serving, or an rc script to start things off when the jail starts up.
So basically, it runs from very easy (nginx config running on FreeNAS), to fairly easy (nginx or Python in a jail).
If you go the nginx in a jail route, you might find yourself tempted to set up a reverse proxy in front of the services as well. Then you'd be able to access things by subdomain or URL path. You could put stunnel in front of that and have everything SSL protected as well.
