Your query on Preen interested me enuf to go to check it out directly with the company:
Preen Garden Weed Preventer can safely be applied on dry, weed-free garden soil, around the established plants that are listed on it's label in the recommended section (http://www.preen.com/newpreen/prdpreen/effect.jsp). Lightly water after application to activate the product.
We cannot recommend this product be applied around a non-recommended plant (Moss and Liverwort), as we have not done tests to make a safe recommendation.
Thank-you for your inquiry and for your wonderful support of our Preen products!

I called Preen's customer service number last year and the woman I talked to said it would turn the moss brown.

I use a couple of techniques on my moss hill, which I cleared of grass three years ago. For annual weeds such as pepper grass, wood sorrel, etc., I use an electric weed whacker at an angle, very carefully skimming the top of the moss - like shaving. I make sure I whack before they go to seed, and blow the debris off when I'm done. Each year I've had fewer and fewer.

I use a potato peeler to dig up dandelions and other tap-rooted weeds - the potato peeler slices through the moss and lets me lift te weed right through it.

Try books on tape or CD when you're hand-weeding!

I think that corn gluten will help reduce any weed seeds from sprouting. It doesn't kill weeds, only prevents the seeds from sprouting. I have not seen it harm moss, but that isn't to say it won't. Try a small area of it. Preen is a similar product, but also contains some fertilizer. Corn gluten will break down into nitrogen eventually.

I'm still in year one of cultivating my moss lawn. I killed all the grass, and sure enough, the moss is reemerging. In some places, there is just a green tint to the dirt. I need to get a battery for my camera to document the growth. I've had a very Zen summer so far, unthreading the spines of zombie wire grass as well many assorted weeds, and carefully tamping the dirt back in place.

I like the carrot peeler tool idea. I've been using a big grilling spatula for harvesting moss. Who needs gardening tools with moss?