I've done a little reading on sours recently and the process you describe is consistent with the practices of several sources I've read. How long do you plan on letting the sour mash rest before blending in the brew kettle?
The length of time and specific strains you use will have a big impact on flavor. Are you going to pitch a Lacto B. culture into the sour mash or just Brett? They have very different flavor profiles since they create different acids and work at different rates. If I'm not mistaken Brett generates Acetic Acid (and a little alc, but that will boil off) and Lacto churns out Lactic Acid.
Either way if you let it cook for a couple days with an active culture you should get something funky. Just keep it pretty warm (90?) and wait till it gets a bit dark and mucky on the top.
Check this site for some good practical reading on sours,
Let us know how it turns out. I've been looking to use this process for an upcoming slightly sour Wit.