"it's possible that some time in the far future the requisite technology will exist."
And it's possible that in the far future we'll be able to warp space, extract massive amounts of zero point energy, and more. In other words, I think it's a cop-out to hand wave about technology for this discussion by saying that it can be done in the future.
"The point of shooting someone with a weapon that can travel that fast is that you can catch them with their pants down."
The problem with this proposal is your target can easily solve this by randomizing the thrust enough, on the assumption that it might be targeted. Suppose you are 1 AU off and fire at where you expect your target to be in 8 minutes. The difference between 0.01g and 0.0101g is 100 meters after 8 minutes, so even a 1% difference in thrust might be enough to miss the ship. But if you're using firing things at 0.99c then your enemy can likely manage better than 0.01g.
You mentioned "all you have to do is wait until the enemy is sitting in port". That "port" is a space station, in orbit. Neptune is 4 hours out. If the station is a 1km sphere, then it only need to move by up to about 20km in any direction to make the odds of being hit be less than 1:100. Neptune is 4 hours out. 20km/4 hours is 0.0002m/s/s or 0.00002g.
For reference, the ISS orbit decays, due to air resistance, by about 90 meters per day. This is easily restored through occasional boosts. Which means that you, as the enemy, are going to need to fire off thousands of these 0.99c bullets in order to hit your target. Where does all of this energy come from?
In any case, with micron sized bullets, you're just going to drill a hole through your target. The exit hole will be pretty much the same as the entrance. Very little of the energy will be deposited into the ship, and it's not likely to take damage anywhere near to the amount of energy you put into trying to hit it.
And it's possible that in the far future we'll be able to warp space, extract massive amounts of zero point energy, and more. In other words, I think it's a cop-out to hand wave about technology for this discussion by saying that it can be done in the future.
"The point of shooting someone with a weapon that can travel that fast is that you can catch them with their pants down."
The problem with this proposal is your target can easily solve this by randomizing the thrust enough, on the assumption that it might be targeted. Suppose you are 1 AU off and fire at where you expect your target to be in 8 minutes. The difference between 0.01g and 0.0101g is 100 meters after 8 minutes, so even a 1% difference in thrust might be enough to miss the ship. But if you're using firing things at 0.99c then your enemy can likely manage better than 0.01g.
You mentioned "all you have to do is wait until the enemy is sitting in port". That "port" is a space station, in orbit. Neptune is 4 hours out. If the station is a 1km sphere, then it only need to move by up to about 20km in any direction to make the odds of being hit be less than 1:100. Neptune is 4 hours out. 20km/4 hours is 0.0002m/s/s or 0.00002g.
For reference, the ISS orbit decays, due to air resistance, by about 90 meters per day. This is easily restored through occasional boosts. Which means that you, as the enemy, are going to need to fire off thousands of these 0.99c bullets in order to hit your target. Where does all of this energy come from?
In any case, with micron sized bullets, you're just going to drill a hole through your target. The exit hole will be pretty much the same as the entrance. Very little of the energy will be deposited into the ship, and it's not likely to take damage anywhere near to the amount of energy you put into trying to hit it.