When I saw this question, I thought “NO WAY”. However, I gave it a try.
To my surprise, I was able to set READ ONLY to model database.
Then I created the database and it was a read only database which didn’t surprise me.
You can see, I was able to to set model to read only and when I created a new database it is read only.
However, after restarting SQL Server , tempdb will not be in READ ONLY.
This is the surprise.
If you check http://msdn.microsoft.com/en-us/library/ms186388.aspx#2 , it says you can’t set model database to read only mode.