What, if anything, is the remedy when a former assistant prosecutor leaves, becomes a defense attorney, and uses knowledge regarding pending cases he/she prosecuted to assist lawyers defending those cases? This is apparently not done for fees directly but in the "spirit of cooperation" among the defense bar and, I suspect, in the hope of future referrals.

My first reaction is to file a grievance; do you think that is appropriate? Is the conduct a violation of the Rules of Professional Conduct?

Maybe some of the knowledge is "Brady material", but if not, there ought to be some remedy. The issue becomes (whether or not for compensation) the lawyer is serving "of counsel" contrary to art. 2.08 or "representing" the defendant contrary to Discip. Rule 1.10 (a), (c). The comment to that rule refers more directly to unfair "access" to confidential information obtainable only through the lawyer's past governmental service. If it is not a violation of that rule, the conduct you describe seems to come pretty close.