Hello.
Yes, it can be done.
Yes, I guess its already done by some one.
But, no, this (working with MySQL) isn't coded in the portal authentication system right now.
That's for the short answer.
The longer one:
On this forum, examples are given how to add the package to your pfSense setup, so PHP can "speak" to MySQL - its just an extension to load.
Of course, you will have to do some PHP coding.
As a starting point: have a look how local authentication with the local databse is being used.
Discover that pfSense uses also a small database motor (I forget the name, you'll find it in the code, in /etc/inc/captiveportal.inc).
A while back, I even managed to add the package that gave me MSSQL (yep Microsoft SQL) PHP support. I could speak with a MSSQL server on the LAN for portal authentication.
The thing is: no-one is gona do that for you, except if you make it a bounty case https://forum.pfsense.org/index.php?board=34.0
edit: go here : https://forum.pfsense.org/index.php?board=2.0 (its where you posted your message).
You will see a thread called : How to Captive Portal Self Registration Using Free radius & Mysql (Tutorial)