I have a mesh that I've warped into a landscape (using a Displace modifier and a depth-map image). I've then gone into edit mode for that mesh, selected the edges running in one direction, and selected "Mark Freestyle Edge".
In this way I'm able to render those edges as solid lines using the Freestyle feature. I'm also able to render them as dashed lines.
Edit: To knock things back to a more basic level, here's a simple mesh with 300x300 divisions and 10:20 (dash:space) lines applied to the edges running left-to-right. You can see that even that has problems, though in this case the dashes remain the same size but the spacing gradually seems to get out of whack.
Update May 5th, 2017: As per TK's suggestion I changed line 1009 of 2.78/scripts/freestyle/modules/parameter_editor.py so that sampling = 0.1 and this was the result: Still some oddness in there, but much much better. Perhaps his permanent fix in the next official release will resolve that. Thanks TK!