Thank you very much for your reply!
I know I am not vectoring the aircraft, but the regulations in force forbid me to provide an instruction which takes the flight out of its route below a safe altitude (read the second part of the Doc 4444, PANS-ATM statement I referred to). You would probably think that it is enough to state something like "Cleared to avoid at your discretion, maintain visual separation from the ground, inform me when you can proceed on course.", but I cannot hand over the responsibility to the pilot, even if he is willing to assume it.
Just to give you an example that takes place every day, imagine you are arriving to an airport along a designated STAR, and at some point, ATC clears you to fly to some direct point or navigation aid in the STAR (no radar vectoring, therefore). The controller cannot clear you below his minima, even if it is cavok and during daylight, and you state that you have ground contact and able to assume terrain separation.
As far as I know, the only chance the ATC has to hand the responsibility of terrain avoidance over to the crew is when performing a visual approach or a approach in contact. But there is not such an equivalence for a departing aircraft, let alone with adverse meteorological conditions.
With regard to your comment about the pilot's knowledge of surrounding obstacles, I don't find it safe to rely in such "procedures". MSA or MVA are published not as references, but as minimum altitudes (as the initial letter says). There have been numerous CFIT accidents as a result of minimum altitude violations by overconfident pilots who somehow lost their situational awareness. It also seems to me strange that airline regulations allow such behaviours.
Wouldn't it be safer and more reasonable to delay the take off until you think you can reach a minimum altitude above which the controller can give you the requested turn?