Dithering - lots of links online, a quick Google search should bring them up. Very easy to do manually - in between each sub, stop auto guiding, move the mount slightly (e.g. a random amount less than 30 pixels in both x and y axes), enable guiding, and start the next sub.
As for noise, check out
this bottom-right corner crop of a dark frame taken on my 5DmkII at ISO 800 x 301 sec after continuous back-to-back shooting. Subtracting it from a light frame doesn't leave a lot of image left! Side note - without dithering none of that noise would get canceled out.
You can get
H-a clip in filters for modded crop cameras. There's lots of into about narrowband online and on here (search for it), but short answer is that very narrowband filters (<= 5 nm) can be good at cutting out light pollution. Some of the nicer RGB filters for CCDs will also selectively block some types of light pollution.
Peter - do you mind posting some of your raw "milky" subs? I just noticed your shooting parameters and unless your light pollution filter cuts out a HUGE amount of light, it sounds like the subs may be overexposed? Ideally, you want the main peak of the histogram to be around 1/3 to 1/2 on the left side.
Edit: I found
these slides about narrowband imaging from one of Don Goldman's talks (i.e. the Don in Astrodon) to be very informative. I'm technically minded, though, so YMMV as they say.