I feel for you John, and for every woodworker who's customer doesn't embrace the standard disclosure, "Wood is a natural material with natural variations in color, that changes over time."
Wenge will work to your benefit in this case, I think, because most people don't see a lot of coloration or grain figure in wenge. It's there, but you have to examine it more closely than most consumers would to notice. So, being a bit heavy-handed with coloring it isn't going to make a difference to the customer, probably. If your customer is different in this regard, let's kick that part around a little.
My initial inclination is to suggest two test pieces. I'd feel better presenting you with a single choice but these two methods will look different, in a way that is difficult to describe. On a high-end piece like this obviously will be, with a challenging finish, I would always (always!) recommend a test piece for approval from the customer.
My proposal is testing these two schedules:
Schedule 1) Color veneer desired black/brown with pigment stain. You can color just slightly lighter than the final color, adding the filler and topcoat will darken slightly. Fill grain with a low viscosity epoxy made for finishing. System 3's product is called "Clear Coat". I'm sure other companies made similar products if you have a vendor you prefer. What you're looking for is water whiteness and low viscosity. Typical bar-top epoxy is far too thick to work in this application, it won't flow into the pores well. Please clear with the epoxy manufacturer the specific lacquer you'll be using as topcoat. In general, lacquer will go over epoxy but it's always safest to check. Be careful if your finishing room is hot, epoxy heat-cures and using it in a hot environment may cause it to start curing while still in the container. If it begins to stiffen up, you should not use it. The most common cause of failure in epoxy finishing is failing to completely mix the resin and hardener. Be certain the two parts are completely mixed before application. Either of the parts that aren't mixed together will never harden and will have to be removed and fresh epoxy applied.
Schedule 2) Color an oil-based filler just a hair darker than the desired final color. Either universal tints or artists oils are excellent for coloring filler. Smear a thin coat on a scrap and allow to dry to check the color. Fill grain and allow to dry well. Scrape/sand back to flat, reapply if needed. The filling is completed when the stock is dead flat, and the pore filler is very distinct from raw wood. Then, color overall with stain that's just lighter than the filler. It will hold on the wood, and be resisted by the filler. Allow the whole assembly to cure well, then apply topcoat. Most anything oil-based is compatible with lacquer, if allowed to cure fully.
If you choose schedule 2, saw your veneer just a little thicker to accomodate removing a bit of stock when you sand back the filler.
On this job, I recommend against using any dye, go instead for an all-pigment solution. All dyes fade, and this designer seems to not be too big on that idea. Pigment only stains are hard to find any more, one that I really like is Woodburst, http://www.woodburst.com/index.asp
. This is an extremely finely-ground pigment in a tung-based carrier. Another option is to use high quality artist's oils, like Winson & Newton "Artists' Oil Color" line. They do, however, dry very slowly.
A concern was raised amonst the mentors regarding these schedules on large pieces. Aside from the epoxy caution mentioned earlier, these products will all behave similarly in time to ordinary oil-based varnishes, or even be a little more forgiving. You obviously don't want to kick off for lunch in the middle, but working at a normal pace should be all that's required.