It seemed like it should be possible to write a script to let you pick an arbitrary file and use it as a texture. That way, you could simply run the script and pick the texture without having to scroll through the textures popup menu to get to "Other".
Unfortunately, it didn't work out. Fireworks seems to perform some magic when using the Other option that can't be reproduced when scripting via the JS API. I tried copying the texture image to the Textures folder before applying it, but that didn't work either.
Get used to scrolling, I guess. :|