I'd still never go for dragonmarks on a first life tank and don't recommend it. The AP and feat are too expensive. That being said, feel free to give it a try and see how it works. To each their own. Whatever floats your boat.
- Displacement has limited use when tanking a boss that has true sight. It can also be gained via clickies.
- Globe of invulnerability only works against spells, not really applicable for tanking bosses. Also requires staying in one place which isn't always attainable.
- Radiant Forcefield ... decent. I personally wouldn't go for that, but hey, whatever floats your boat.
I'd rather use those AP in other ways, to get hit points or PRR or increase intimidate.