"But as implementers aren't lawyers in a more powerful position to influence the outcome?"
Yes, i agree the answer to your question is "yes". But the question was not "do lawyers play a key role in the development of law that rules society?", the question was "are lawyers social engineers?". Those are two radically different questions, an affirmative answer to one implying much more than the other.
Also, if a lawyer is advising a congressman on the articulation of a particular bill and he presses, owing to his own personal beliefs, the congressman to proceed in one direction rather than another he is doing so not qua lawyer, but qua any of a number of professions who have influence over the congressman. That is, in his capacity as a legal advisor to the congressman the lawyer is only being a lawyer in so far as he assists the congressman in articulating his (the congressman's) own independent vision of the bill. The instant the lawyer begins to press his own opinions on the congressman regarding "what is right", he ceases to be the congressman's legal advisor and instead becomes a private citizen advocate who is taking advantage of his position of influence. Lots of people besides lawyers come to mind in their ability to influence lawmakers in a similiar manner - lobbyists, campaign donors, etc. I don't see a lawyer acting outside the bounds of his profession (legal advisor) and influencing the congressman with his personal opinion (rather than his professional advice) as any more of a corrupting influence than lobyists, donors, etc.
bobefette - in so far as citizens develop a consensus of opinion and elect officials who concur with this consensus of opinion and said officials then enact laws that reflect this consensus of opinion, yes - the citizen body is engineering social change.