[kreios]

I think it's safe to say that, no matter how much armor you pile on a space craft, the sheer energy produced by any particle moving at relativistic velocities will swiftly penetrate. If enough mass of particles hit, the space craft will get vaporized, no matter what.

That's definitely true - yet that requires particles to hit at relativistic velocities. I believe that restricting usual relative velocities to less than let's say 150km/s will not (kinetic energy of 400x its mass in TNT). That's still less than a nuclear weapon, and has to actually hit.

Firstly, even though you kind of resist the notion, an aegis-style point-defense is just too useful to pass up. I'd put in a combination of automatic turrets sporting high-velocity rotary cannons, and maybe some BSG-style turrets that fire AAA that puts out an ungawdly amount of shrapnel.

Oh, that's definitely not a something I'd resist. I should probably clarify that:
I fully expect (and want) spacecraft to mount point defense. What I do not want is them mounting what amounts to a laser rifle on an articulated mount. Instead, they should mass a few tons to be effective.

Secondly, I'd pull in a trick I read in one of David Brin's Uplift novels. In that one, a dolphin-piloted starship destroyed a pursuer by dumping much of the water in its habitat-tanks in a cloud behind, as it whipped through a sling-shot orbit around a gas giant. The water immediately froze into a cloud of fine ice crystals.

The vector of the pursuing craft, also in a sling-shot orbit, took it straight through the cloud of ice, and it promptly vaporized.

Interesting. They'd have to have pretty different trajectories to generate that relative velocity.

Is that actually a possible tactic, both offensively and defensively?

Let's assume for a moment we want to use this tactic of sandcasters to project a "wall" of particles across one 10-mile hex, each of the grains being one gramme. To avoid a missile to penetrate this defense, we need roughly five or so particles per metre, for a total of 80,000 grains. That's a total weight of about 80kg, the equivalent to one 12cm EM Gun projectile. That's the absolute minimum; the actually required material is going to be much more since the grains won't be equally distributed, move outwards, etc. And many, many more particles against laser fire.
It's even worse when we're looking at three dimensions, where you'd need to use at least 6.4 billion grains, for 6.4 tons, to generate a ten-mile by ten-mile barrier.
Both suggests that the use is for an already-guided projectile to use this to ensure a kill. This is actually how I imagine proximity detonations to look like - the projectile disintegrates into many particles a second or so before impact, spreading into a rough cloud.

Which brings me to another idea: Use Hitting the Wrong Target rules (B:389) to check whether you hit something in the same hex, too. This would probably be bounded by 9 plus 4 for proximity detonation instead of just 9.
Thanks!