Usually those lids are coated with a polymer to prevent that metallic taste from transferring. Lids that have been used a lot (and washed a lot) might not have the coating anymore, but new ones should. The same kind of coating is used in aluminum beverage cans, for the same reason.
Again, for the same reason, I'd still only fill them so the mead doesn't touch the lid.
It should be remembered, that one of the biggest reasons why even industry only uses plastic to contain alcoholic bases drinks, is because it is a solvent.
Of course, it does depend on the strength, ABV-wise, as to how much solvent action occurs. Here, it's mainly beers and ciders (hard cider, I think you'd probably call it) that is sometimes sold in PET, but the very nature of the drinks is relatively low ABV%, and that does reduce the solvent action/extraction of plasticisers etc. The "wine in a box" stuff? well that's also meant for reasonably quick turn around, reducing the possibility of off flavours or other forms of plastics contamination.
Hence, the "real" producers, use glass, for long(er) term storage as it's inert in the presence of alcohol. Allowing for long term contact etc.
So if the OP is considering longer term storage, I'd keep the mead etc, away from the resin on the metal and/or metal.......
Just my 2 pence worth.....
regards
fatbloke
