Hi Archdaly
I spent a long time today trying to find out if this could be done. The linear material tool seems to be controlled by a vector script plugin, so if you know anyone smart enough to be able to edit it, then it might be possible. Unfortunately I am not clever enough for that, but I do have a slightly crappy workaround.
I only really use the tool for drawing plasterboard details (gyp. bd.) so needed to change the hatch as it is really stupidly heavy lineweight. The best solution I can find is to basically turn the hatch off by setting the density to zero and then giving it a fill with a normal hatch (I used 'sand').
As I said, it's a bit crappy but worked for my purposes. My computer seems to run a bit faster too, in the Vectorworks help it actually mentions that the stipple fill it uses may lead to slow rendering.
Best of luck,
Matt