The problem with the CD checks is that it would cause problems for people that have the Steam or other digital versions of the game. I think a better idea would be a check for the Unreal .exe and a few other files so that people that have the digital version or plan on only having the digital version in the future (like me) won't get left in the dust if this is released.
As for the legal problems; have you blatantly told them that you have no intention of obtaining the source code? All you want is the beta files (like textures and models), .exes and the versions of UnrealEd attached to the betas.
No I didn't blatantly tell them that, but I thought it was clear. I mean there is someone who even got the sourcecode of Unreal by asking them and signing stuff, but I did not mention that. Why would they think I want sourcecode? Anyway Epic doesn't seem to have these files, or maybe has but hidden somewhere deep...