Can Behavioral Health deny me getting a second opinion off post?

Not Normally, but it will come out of your pocket.

They can't deny you going to see an out of network provider - they just don't have to put in a referral. So if you go without a referral, you will be charged as Point of Service by TRICARE and you have to pay for it. If you want a second opinion and they won't put in a referral, just go and pay for it. If you can't afford it, then don't go.

They can't. But as stated, you'll have to pay out of pocket.
Or you could always to your friendly unit PA, explain whatever the situation is, and that you would like a civilian second opinion, he/she/it can order the referral.
I did it while still in.