If you're only really needing these settings, why not save yourself some cash and hassle and just move the NG2 middle pickup to the regular combustion neck/bridge configuration and pick up a new pickguard.
No need to change the wiring or add a switch etc. and it will give you Bridge, Bridge/Neck - series, Bridge/Neck Parallel and Neck solo.
Or buy a standard Combustion and add the DG preamp later if you wish?
Probably be a cheaper option...?
Also yeah, watching this upgrade thread with interest - been talking to Waltsdog about doing something similar myself so the more ideas, the better.