I'm not sure all log book entries must be signed. The FARs specify that instruction given in another jurisdiction by an appropriately qualified instructor under that jurisdiction's rules counts towards an FAA certificae. Nowhere does it say in that section that the logbook must be signed. If that country doesn't require a logbook signature than that's part of the rules for that country.
Where it does say it must be signed (to count as instruction) is in another section and refers to an 'authorized instructor' (sic). The FARs seem to be quite consistent that 'authorized' refers to an FAR licenced instructor exercising the privileges of an FAR instructor rating eg the requirements for flight reviews etc