I sent my Marchesini magnesium rims off to a specialist that deals with magnesium components in the aviation industry.
Well, as they commented, there isnt a hell of a lot of magnesium in them compared to 'true' magnesium aircraft rims. The cost of NDT, corrosion cleanup and paint prep was very little and well worth it.
The magnesium content in the Magteks is even lower than the Marchesinis - hence them being sold for use on a roadbike where it is unlikely that the owners will strip and NDT them every year as well as subject them to all sorts of road conditions.
Comments on them seem to vary from the ignorant 'they will be ok' to the paranoid 'send them to Marchesini for no one else on the planet can fathom what to do with them'. The aviation industry is very familiar with dealing with magnesium.
I think that it was Robert Dunlop that had a Marvic rim separate from the spokes of his RC30 while riding at the Isle of Man...