I've had spotty results with seed balls. Generally they are best suited to soil that has been recently disturbed and without existing groundcover. It's not to say it can't work but you may want some additional grow medium to ensure establishment. That may come in the form of a couple handfuls of
compost accompanying the seed balls (I've taken to planting trees and shrub tree seed balls rather than throwing them on the surface.
6 to 12 inches of biomass is fantastic for the system as a whole but the short term will pose its own challenges. Firstly, if the material is fresh-ish (one year) then the quantity is going to suck a ton of nitrogen from the soil before it starts giving back. this is a natural part of the decomposition. what you may want to do is select species that are hardy to poor soils first. These plants (pioneers!) will support the soil food web you will depend on, encourage fungal growth and consumption of the chips, and blend the soil below and the mulch you've added. I would go for nitrogen fixers (and their accompanying inoculants) and anything with tap roots to start as they'll likely have the structure to support themselves on this medium. Shallow rooted plants may find the 12 inches of mulch hard to spread in with adequate support. It'd be a shame to lose a bunch of 4 or 5 year woodies to a serious wind storm.
The wood chips are going to tilt the soil food web toward a fungal dominated system so woodland and edge perennials will love it but finicky prairie perennials and annuals may find it inhospitable. If you need some help selecting species let me know and I'll do what I can to help.
We're all experimenting and we all benefit from communities like this. The best answer I can give is in the form of a request. Please document, test, scrutinize, and share your experience with us. We all have a hand to play in furthering our collective knowledge-base!
I look forward to hearing results.