The possibility that Steve suggests below is probably real, but remember that the electronic ignition will only advance when the manifold pressure - and hence the cylinder pressure - is lower than the max (only at high altitude or part throttle).  And the electronic ignition will only advance 5 or so degrees more than the mag anyway.  In 5 or 10 degrees the cylinder pressure will not rise much.  I would guess that at higher engine speeds the burn at 10-degrees after ignition is less than 10% complete and probably more like 5%.  It is my judgement that there isn't much to worry about, although you are correct that the life of a coil depends on the peak voltage it sees, which goes up with pressure.  I'm sure it won't rise to the point that the mag won't fire, though (of course, how would one know?).  There isn't much concern related to higher altitudes, because under those conditions the cylinder pressure is lower than at sea level.  Since there is evidence to suggest that a coil expected life goes down as the peak voltage goes up should all of us that have one mag be worried?  I don't think so, but I don't have firm data to back it up.

As I type, though, I think of another issue - say the cylinder pressure rises enough so that even though the manifold pressure is 50% lower than at sea level the firing voltage is not that much less.   The coil will still fire, but the likelihood of cross-firing in the distributor is higher.  The idea is that the post spacing in the distributor is set assuming the cylinder pressure goes up and down with atmospheric pressure and therefore there will never be a time when the air pressure inside the mag is insufficient to prevent cross-firing.  This is what the turbo guys worry about and the cure is to pressurize the magneto, not possible with an NA engine.  The symptom would be a rough-running engine at high altitude and full throttle and I've never had that experience.