I've seen them in otto links, so i assume they could be put in anything. The varitone was in use on selmers in the 60's, and didn't have a particularly long run, so your mpc is probably somewhere in the mid 60's-70's. my understanding is that the research team with varitone found that the best place to put the pickup was on the neck, however players that didn't want to permanantly alter a neck would have them installed on the mouthpiece, since mouthpieces are much easier to replace and (for the most part) much less expensive than a neck or entire horn.
probably the best way to fix it is to get it plugged- that's how i hade one removed on my mark vi neck. the repairman pressure fit a slightly tapered brass plug and added a little glue to seal it. it's been fine like that for 4-5 years now. check out this too:
http://www.jgerber.com/serviced/link_hole/index.htm
i assume that hole is from a pickup, it looks like the right size and was certainly made on purpose.
hope that helps- mike
