R' Moshe (igros y.d. 1:57 - i mentioned it in the last page of my sefer mayim rabim on mikva) has a chiddush regarding tevilas keilim of electrical appliances. After proving the the electrical element and wires are considered a separate keli since they serve an entire different function than the rest of the keli (such as to heat the contents rather than hold the contents). Regarding the point that the 2 parts of the keli are attached, and water is not penetrating that spot, R' Moshe says that it has a din of beis hastarim that doesn't need to come into contact with the water or even need to be fit to come into contact with water (unlike a person where beis hastarim needs to be fit for bi'as mayim).
It seems from rashi that he rejects this sevara. The gemara says that all agree that for tevila the tongue is considered beis hastarim and water does not have to touch the entire tongue. Rashi asks, that since part of the tongue sometimes protrudes out of the mouth, the front end of the tongue shouldn't qualify as beis hastarim and should have to come into contact with the water. Rashi explains that since the back part of the tongue is inside the mouth, we never find the concept of "tevila b'miktzas". therefore either the entire tongue requires contact with the water or the entire tongue does not need contact with the water - we assume the latter. Based on R' Moshe's suggestion that the point of contact between the container part of the vessel and the electrical element does not need contact with water, and it is sufficient to dip the kli half way, that should also be a possibility with the tongue. Maybe the front end of the tongue needs bia's mayim until the part which is always in the mouth and would qualify as beis hastarim, as well as the point connecting the front half and back half which would also be beis hastarim. Why then does rashi insist that it is impossible to have tevila b'miktzas?
Perhaps a distinction can be made between the point of contact between what is essentially 2 keilim, each with its own function, and the notion of doing a partial tevila on the tongue which is one unit.