A few points.
1. I suggest that with the disruptions from the failure that caused the emergency descent still current, that you don't really have the time to do nice little theoretical calculations to arrive at a target VSI that just satisfies the regulations 4 minute time limit. The idear is that you get your passengers down to a density altitude where they can stay alive and not having extra oxygen, 4 minutes is deemed to be the maximum time to do that without compromising their health too much....(Think of all the extra paper work if you stuff up the calculations !)
2. The high MOCA is the limiting factor in your descent options and it is 'nice' to have a comfortable margin from 'terra firma' in such a situation ...... especially if its 'stato-granitus' where you can't see the stuff but know its down there somewhere !
3. Drift down charts cover N-1 not depressurisation with a different set of criteria, ie. stay UP as long as possible (or minimise the descentgradient).... NOT get DOWN as quick as possible (well within 4 minutes anyway) but the charts do provide information on terain clearance which someone has gleaned and tried to apply to the emergency descent situation.
4. If you are approaching the most critical point of the crossing when the 'event' happens then THEORETICALY, by calculating and applying the target descent rate to meet the 4 minute requirement and doing it at barber-pole speed (conditions permitting) by adding power ... this will result in a descent gradient shallower than at throttles closed and thus the maximise clearance from terrain underneath.
5. This may or may not keep you at or above the drift down curve ... nice if it does but I dont think your going to have the time to check it out.
6. Can't you choose a route where the MOCA is below 13,000 then you don't have to worry about it. Plus, if its VMC then descent below the MOCA is possible, may be even desirable if its >13,000. Fly along the valleys at 10,000 for example.
7. But like I said in point (1) do you have time in such circumstances do do all this mental gymnastics in the middle of an emergency proceedure. I think not ! 4 minutes goes very fast in such a situation.