Alienware 17 R4 Repaste with Thermal Grizzly, can a Dell tech do it for me?

I'm unclear how the LCD replacement factors into this, but if you're asking if you can just buy a bunch of parts and contact Dell to send someone to do the installation work for you, then although I'm in the US where the warranty terms admittedly may be different, I seriously doubt it.  Technician service provided under a warranty agreement is for repairs, not a resource you can request for your own desired modifications even if you were willing to pay for it.  Now if you had a technician coming anyway for some other warranty service that would require replacing the CPU paste in order to complete, then you may be able to get the technician to use the paste you provided rather than the paste that Dell would have included.  I'm sure the official policy is for only Dell-provided parts to be used (and in fairness, I can see why), but you never know.

As for voiding the warranty, it's true that working on your system doesn't void the warranty (assuming you don't damage anything), but using an aftermarket product is another matter.  In that case I would imagine that if you subsequently had a failure that even MIGHT be attributable to your use of an aftermarket product, then the warranty claim would be denied, and it wouldn't matter if your aftermarket product was supposedly better than whatever Dell provided.