Cool. Here's my thought: If you're really intent on making a lambic, then get some lambicus bacillus yeast.
Personally, I wouldn't make a lambic...just make a belgian dubbel or a wit, then add cranberries to the secondary vessel. I made a cranberry dubbel awhile back, and it's excellent. The key, I think, is using Wyeast's Abbey Ale yeast, which is magnificent stuff! So freakin' spicy, you'd think that you had put cinnamon and cloves in that *****! I love it. Anyway, I used lots of belgian aromatic and special b, along with some other cool stuff. I'll post the recipe if you want. Also, I'd suggest using two pounds instead of just one...get the ocean spray bag from the supermarket if you have to. Otherwise all you'll get is a weird sourness, but the actual cranberry flavor will be very muted.
How to deal with the cranberries: as you want to make sure they're sanitized, you'll want to heat treat them prior to adding them to the secondary. Wait until primary fermentation is done. Take the cranberries and steam them on a stovetop double-pot steamer. Make sure to put plenty of water in the bottom, as the steaming water will also be used to boil. So, steam them for 5 minutes to get them nice and plump. Then, pull apart the steamer and pour all the cranberries into the boiling water below. Using an immersion blender, partially puree the berries so that you end up with a watery/syrupy liquid with some cranberry solids retained. You do NOT want to completely puree them! Simmer this for 10-15 minutes, cool, add to your empty (sanitized) secondary vessel. Then rack from primary on top of the cranberries. Steep like this for a week, then rack off into a tertiary vessel; the skins have alot of tannins that you want to avoid. After a week or two(or three or four if you have the patience) in tertiary, all the cranberry solids that were transferred from secondary should have settled out. Time to bottle. Based on taste testing, you could also choose to add a bit of cranberry extract at bottling, which is available from most HBS's. Austin makes a good one.