A pilot who can not legaly fly the aircraft in IMC can not legally assist the pilot under the hood if the flight is in IMC.
A pilot without a valid IR or IMC-rating can legally "fly the aircraft in IMC". He cannot fly as pilot-in-command.
Please tell us where the requirement to maintain an adequate lookout is not required in IMC.
Your reference to "lookout" is not relevant here. A pilot flying by reference to instruments during an operational IFR flight is undoubtedly not maintaining a good lookout. The requirement is not to "lookout" but rather to "take all possible measures to ensure that his aircraft does not collide with any other aircraft". That has to be taken with reasonableness and in context -- the only way of satisfying the rule literally is to leave the aircraft in the hangar.