> A military target should make a point of either being hardened enough that it could take such a hit without serious damage
Well, with a foil weighing just a few micrograms you could get a yield equivalent to several kilograms of TNT, assuming an impact velocity of several thousand km/s (and thus significant fusion yield). Increase the impact velocity by a couple of orders of magnitude and you're looking at about a ton of TNT.
> or unpredictable enough that you simply couldn't be that sure.
How? In space it's not like you can just change direction at will. Doing so requires a lot of fuel and a lot of mass. You yourself are saying that your missiles would have to be light and they'd fire their thrusters once to head towards their target.
> Beyond that... the laser would be detectable. It would both make the "foil" detectable which would mean it could be intercepted by something else.
Not necessarily. Unless you're directly in the beam path, it's hard to detect lasers. And if you're in the beam path it's already probably too late.