I've been looking into non-hop bittered beers as well. (sometimes referred to as gruit ales) There is some good info on Mugwort and other gruit herbs at http://www.gruitale.com.
You may even want to look at their posted recipe for Mugwort Anti-Imperial Stout.
I have brewed two batches of gruit at this point. The first came out so bitter that it was not enjoyable... (so I brewed a malty sweet Scotch Ale to pour 50/50 with it.. which was much better). My 2nd iteration on that recipe, using Greater Wormwood (a stronger cousin to Mugwort) and Bog Myrtle (aka Sweet Gale) has come out *much better*! Here is a link to my recipe for Ye Olde Gruit Ale.
Last bit of advice, either brew a small batch if you are unsure of the quantities to use, or take the lower end of any ranges you find. Also, any herbal or spiced beers will benefit from an extra 3-4 months of aging. If it is too strong at 8 weeks in the bottle, age it another few months and try again. They mellow considerablly at 6+ months...
Good luck & Enjoy!
p.s. I saw a recipe in 'Sacred & Herbal Healing Beers' which called for 2 oz of dried Mugwort in the 4 gallon batch. The above recipe for Mugwort Anti-Imperial Stout calls for 1.5 oz in 4.5 gallons.