Yes, they will survive, possibly, since all zoas and palys aren't a like, but I would never risk depleting those polyps of light, here's why.
I once left some frags in a bucket after fragging for almost 2 weeks, they were still alive when I found them. Just because they will surive, doesn't necesarily mean you want to intentionally subjest them to light deprivation, especially in your case. Why? You will stress your polyps out big time and you could very well and possibly kill them, POSSIBLY. You have 6 -54 watt T's, which is what they are accustomed and acclimated to. You're going to subject them to no light for 5 days, then you are going to blast them with MH lighting. Good lighting, to no lighting, to the most intense lighting on the market. Will they surivive and thrive, yes, possibly, most likely. But do you want to risk that, I wouldn't. I like to error on the side of caution. Now if they were common easy to find and replace inexpensive zoas, yes, I'd do it. But not with P's, R's and palys. I'm no lighting expert, but I would hate to see you lose them. If you can, why not borrow a comparible fixture from a fellow reefer in your area. It's done everyday. Is it possible to explore some other options to secure the funds? I just wouldn't risk my polyps at the cost of possible bleaching from the blast of your new MH. Of which you should slowly acclimate your corals to the new more intense lighting from the MH's.
As Whodah said, not all zoas and palys are a ike.
"does it stand to reason that everything else being equal (no oxygen deprivation, adequate flow, etc.) that a zoanthid could survive 3 weeks without light if it could remain closed for 3 weeks? does a closed zoanthid still benefit from light perhaps by penetration through it's stalk?" That's a great question too.
Unprofessionally, I'd say yes, they do indeed benefit from the tank lights. I believe that it has been proven that zoas do fed since they have a gastral intestinal tract, however lighting is also proven to be their main source of nutritonal uptake. That said, Whodah's point is a valid one. Do I have the proof, no, do I believe that closed zoas in a lite system will fair better than closed zoas without any light, yes. I wouldn't call it proof, but zoas are known to cup/close up for periods of time as a means of externaly cleaning themselves. I've witnessed them closed for up to 6 to 9 days before opening back up without any signs of decline in character, color and overall appearance.
Just my 2 cents.
Mucho
need to turn that timer off heh.
