PLD can Shelltron back to back yeah, but you really shouldn't. You have to build your gauge back up, it amounts to about 21 seconds of autos if you don't need to step away from your target. Damage reduction only lasts 8 seconds where the HoT lasts 12, so you want to keep those both working for you. You can, however, use Shelltron pretty much every 20s resulting in intentionally stacking defensive cooldowns, which actually increases the value of the regen that comes from Shelltron. The math here is a bit complicated and I... don't feel like doing it this time. But it means the HP value from the regen builds up to a significant number. The PLD has a regen totaling a potency of 1000 every 20s, roughly 30 thousand using my level 99 gear and tomestone accessories. DRK was given a 1200 potency once every 2 minutes. PLD has a 400 cure potency every 3rd GCD in AoE, then 400 cure potency on 4 GCD in Requiescat making 1600 over 4 GCD. DRK has a 40k shield every 15s but only 2 opportunities to fill that gap with any sustain. I really think a 100 or 200 potency regen for 12 seconds on Oblation solves this issue no problem, as well as a 10% mit on Dark Mind for all damage not just magic (Never forgive that Camouflage is just HW DRK Dark Dance reworked, buffed, and given to a different tank). 6000 heal potency over 2 minutes (not calculating Abyssal Drain because it's target dependent) will still not compare to the PLD with it's 2 minute heal potency total being 29,200ish (idealized AoE scenario) while also giving PLD a 1000 potency shield on top of it all. Baring in mind that Ramparts can amp any 20s window of that healing as well, and that, of course, those heals can crit, and over the course of a pull, they will.. (Why 2 minutes? Because that's when all cooldowns would be equally "reset", I do realize how infrequently you'd get all 2 minutes out of a single pull, but it's the simplest way to display the math, as well as using PLD because it's pretty baseline with what a "healer tank" should do)
That said, the reason I point out it wouldn't be healthy is because DRK would then just not take damage at all, even Warrior has to actually take damage. A lowered cooldown on TBN could work but adding the MP regen wouldn't. Outside if Delirium, it takes 6 GCD's to recharge 3000 MP for TBN, which is basically a 15s recharge anyway, so you'd have a burn mechanic for your MP, but again, once the shields are down, pretty much any damage you take sticks on you forever.
EDIT: Interestingly if a single 1000 potency heal on my gear is just over 30,000 HP then by that math PLD would heal around 300,000 hp with no crits or ramparts, which at 40k hp shielding from TBN is 7.5 total TNBs which also takes 2 minutes to burn through on a 15s cooldown if used entirely off cooldown. Obviously round up to 8. So if the healing without crits and without ramparts is actually the same as TBN, surely something is to be said for the extra damage reductions PLD gets from Bulwark, the greater value from HS compared to Oblation, and the passive blocking that does happen regularly? This kind of math is why TBN just doesn't work, it's too short term and with nothing to fill in the time inbetween when damage is properly taken. It's more damage taken with less health to bump survivability. The math is there, I don't know why the devs allow this to continue. Abyssal Drain also isn't really a heal to me, it's more of a "free bounce back once a minute" in pulls, but this could be attributed to my way of thinking with reliability and consistency and pressure to use it in burst windows regardless of current HP. That's why I haven't considered it into my numbers.