Just bought a season ticket guest ticket for a friend at work. He's a neutral (supports some obscure Midlands non-league team) but think that was literally the only ticket on sale on the site this morning. Unless more exchange seats pop up it is well and truly sold out.
Definitely on sale to season ticket holder guests and as others have said plenty of availability this morning. Just remember it is only 1 guest per season ticket holder so you'll need to first make sure Stat Junior is in your Friends and Family then assign 1 ticket to yourself and 1 ticket to...