Here's an odd idea that might turn into something useful. A group at Berkeley (spanning both the chemistry and physics departments of Cal-Berkeley and the Lawrence labs) have reported a method for encapsulating organic molecules and releasing them inside a reaction when needed.

What they do is form microcapsules, small polymer spheres, from branched acid chlorides and amines. That technology is already known, but in this case they're also incorporating carbon nanotubes inside the capsules, as shown in the photo. If you do this from a solution of some reagent of interest, you now have it, the solvent, and the carbon nanotubes wrapped up in small polymer beads.

And if you irradiate these things, the carbon nanotubes heat up rapidly, causing the microcapsules to break open. There's the control mechanism. They've demonstrated this for reactions such as the "click" triazole formation and for olefin metathesis. You can follow the reaction progress, and it goes stepwise, further every time you hit the solution with a near-IR laser, and stopping until you do it again and release more coupling partner.

The limits of this system, so far, are (1) that the microcapsules aren't compatible with the full range of organic solvents, (2) that heat-sensitive reagents probably won't do very well in a system that require local heating to burst the capsules, and (3) that you eventually have to clean out (presumably by some sort of filtration) the capsule and nanotube residue after things have burst. But some of these can be addressed in further rounds of improvements.

For example, there must be different sorts of polymers that can form these beads, for one thing. And if it's possible to encapsulate things on the surface of a larger sheet of solid material, you could just dip that in and pull it back out when you're through, which should cut down on the capsule residue. (That would also allow you to quantitate how much reagent you've released, by following the surface area of the sheet that you've irradiated with the laser). What would really make this something to see would be if a way could be found to release different sorts of capsules at different wavelengths, selectively. . .