Jack,
Those Emeralds are nice. I've still got a pair that I built from kits approx. 3-4 years ago. I had the kits sitting around for a few months and as soon as ACI released the Sapphire III LE's I decided that I'd finish my Emeralds the same way. When I showed Mike Dzurko the picture he told me that they were almost exactly like the Emeralds, which I hadn't even seen yet. Mike's truly a class act. In many ways he's responsible for my being in business for myself now. If I hadn't known about his speakers and kits I probably wouldn't have started building speakers.
Anyway, onto the sub. We usually stain/dye red oak black but I think that using a semi-gloss black paint might be the best way to go. It's much quicker than the 2 step stain/dye process and doesn't need to have a top coat applied so it's less labor intensive, and therefore, less costly to the customer.
I'm more than likely going to use a round grill. I think it will be easier to stretch the grill cloth over the frame and will look better than your typical rectangular/square frame. I've thought about using magnets. I've got a bunch in the shop so I could definitely try it out. Now that I think about it, one of Mike's guys contacted me earlier this year asking me where we were getting our magnets from.

If you've got any more questions feel free to PM me or email me. Also, I'll be in the shop tomorrow from 9:00am until probably 2:00-3:00pm EST tomorrow if you want to speak over the phone. The shop number is 706-557-8694.