Is it possible to use dry ice to separate THF from THF/MEK mixture by freezing the MEK and pouring off the THF like separating water from alcohol?

probably not

Are the BP's really close?
If not distillation should work.  Take appropriate precautions for peroxide formation.

Boiling points are very close.  Melting points are THF 108 and  MEK around 80 below zero celsius.

According to my MSDS's, MEK melts at -86.4*C and boils at 79.6*C, while THF melts at -65*C and boils at 66*C.  You should get away with fractional distillation, unless of course there's an azeotrope...

If you are indeed planning to separate THF from anything else by way of distillation and you aren't aware of the dangers of doing so, such as not letting the distillation flask get too close to dryness as it could be a fatal mistake.  As well as the formation of peroxides which could lead to an unexpected and unwanted explosion.  It has properties similar to Diethyl Ether as far as dangers.  There is a chemical that can be added to it to stabilize it though, but the exact name eludes me at the moment, something like BHT or something like that.  But your local chem supplier should carry both and shouldn't be too concerned about you inquiring or purchasing it from them.  It ran under $40 for either 2 or 4 gals I don't recall...and this was within 2 yrs ago.  But like I said read up on its dangers if you are very knowledgable on safety precautions needed to work with it. Don't bee a Bumble BEE, bee a safe Bee ;-)   Pyrex out buzzing round'

BHT stands for butylated hydroxy toluene, or more specifically 3,5-di-tert-butyl-4-hydroxytoluene.