LN2 has a specific heat of vaporization, Cvap, and a specific heat, Cp, as it warms from 77 K to 233 K.  Multiply the mass of LN2 injected by Cvap, and by Cp(delta-T), add the two numbers, the result is the amount of heat absorbed by the warming of the LN2. 

Don't mount a LN2 dewar on a shaker head, if you shatter the vessel and spill the contents, you're gonna have problems.  Modulating (pulsing) slugs of Chemical pumps LN2 directly onto your device is probably going to cause a lot of frost build up too, unless you take care to work in very low humidity.

Have you considered using dry ice?  Pack a bunch of dry ice chips into a styro box, and monitor hardware temp. as it cools down, start testing when the temp. gets to -40F (remove the box and blow off excess chips at that point).  Concerns for frost still apply; melting frost dripping down into the shaker table may not be a great idea.

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