Risk and opportunity. These are two factors that need to be considered. Whilst the risk that the two occurances may represent is severe one must keep in mind that the opportunity is somewhat limited. For instance, to the best of my knowelege, Paul MVHR systems have not failed during 15 years of opperation furthermore electrical power outages are realtively rare.
Wood burning stoves, if they have a balanced flue, pose a reduced risk - only if their airtightness is not sufficient do they pose a real risk. The seals could concievably fail at some point, and there could be a power cut, but I find myself asking at what point is it likely that these two occurances will occur at the same time. It is easy to compound risks on top of one another but we should be mindful to retain perspective.
..... That said PHI have developed recomendations relating to leakage from wood buring stoves. They recomend that in the event of leakage the MVHR is automatically switched off.