You can circumvent that (to some extent) by using higher resolution height map to generate a normal map, then scale the normal map down to final size (which is the primary reason why I've been doing height maps at higher resolution than the final normal map resolution basically from when I first started fiddling with normal maps).
I did a little testing, and indications are as follows:
-If you have a height map with 1 pixel lines on it, you apply normalmap at 4sample or 3x3 setting to generate a three-pixel wide groove with sloped edges and flat bottom, then downscale that by factor of two at Linear setting, you end up with a nice, two-pixel wide groove without the flat bottom. After that it's simply a matter of enhancing contrast as required on both red and green channels.
-If you have a height map with 2 pixel lines on it, you apply normalmap at 4sample or 3x3 setting to generate a 5-pixel wide groove - 2 pixels on each edge and 1 flat pixel on the bottom of the groove. Then you can downscale by factor of 4 with Linear setting, and you end up with 2-pixel wide groove on normal map.
Finally, probably the most impressive and useful magic trick:
-If you have a height map with 4 pixel wide lines on it you apply normal map at 4sample or 3x3 setting, end up with 6 pixel wide grooves (2 pixels for sloped sides, 2 pixels wide flat bottom on the groove), downscale on Linear setting by factor of 4... and you end up with 2 pixel wide grooves on normal map, the thinnest that can exist.
What this means is that if you have a height map drawn at intended resolution, and need the normal map details to be sharper, you can upscale the height map by factor of four (preferably with "None" setting for scale-up, which preserves all the sharp edges of one-pixel lines and turns them into sharp, four-pixel lines), apply normal map and then downscale back to intended resolution...
Of course this works ideally only for horizontal and vertical lines. Diagonal lines are a bit trickier and results are not always optimal - but then again, diagonals are always a problem in textures due to aliasing effects and you can't apply anti-aliasing without thickening the lines, and if you thicken lines on texture you end up needing to use a higher resolution texture to keep the visual thickness of the diagonal lines acceptably thin. Problem is, of course, that at the resolutions we're talking about... there's not much budget for further upscaling, is there?
Thankfully, the post-processing anti-aliasing will hopefully mitigate some of the aliasing effects. And, even more luckily, most of the smallest details on the ISD seem to be horizontal and vertical lines... so this upscale-heightmap-apply-normalmap-downscale magic trick SHOULD work.
Hopefully it'll save you some work re-drawing normal maps by hand, but if it doesn't provide satisfactory results, there's always the bruteforce hand-drawing method available. Worth a try regardless, I'd say.
Pictured: Progression from one-pixel wide line on height map, to three-pixel wide groove on normal map, to two-pixel wide groove on a downscaled normal map. Testing done with GIMP.