They can be dragged with an adapter in hauled mode. 46100 (assuming its air braked) could have rescued the ailing IET. Now that would have been a photo!!
I think the lack of forward thinking, and the "cant do that" attitude that seems to pervade everything to do with railways these days exposes problems which spread way beyond Dawlish disruption.
Consider the facts:
Yesterdays waves, whilst significant were not beyond that experienced several times during most winters, so if trains are not sealed against sea water from waves they will fail. Sometimes a rogue wave will occur that is much higher than the average, these cannot be forecast currently, which will catch out a unit sooner or later.
So ideally you have a solution which doesn't fail under conditions which are to be expected several times during most winters. However sea water and electrical systems really do not mix, so having a solution which is fully seawater proof probably isnt deliverable.
Given that failures are possible (likely even) a reliable method of recovery should be provided. So a locomotive which is sea water proof as a rescue locomotive is essential.
From an operational point of view surely some sort of adapter vehicle needs to be stationed nearby, that can be hauled by whatever is available, quite possibly a 'heritage' locomotive, even a steam driven one, or maybe a diesel mechanical shunter. this would be a low speed move. The main requirement being that the rescue loco doesn't fail when getting covered in seawater.
Failing to address this problem just seems like a cop out, we know it will happen, the logic behind using the same type of unit to rescue a failure in this situation just defies common sense, and will result in what happened yesterday, multiple failures.
This problem will repeat year after year. Divertng the route would be the best solution, but that will not happen, so contingency planning needs to allow for failures and a reliable recovery method. If the easiest solution is a hired in loco from the Dartmouth railway with a translator vehicle then make it happen.