Small update:
Here are two last shots of the underside before I start on the top side rigging (and posing the aircraft for belly pictures becomes a challenge):


Clearly I have decided to go the oily route for my Eindecker. Virtually every in-service photo I have seen is of an grubby, oil-stained, muddy mess, and I have seen the mess a rotary engine makes on reproduction and restored aircraft. I figured this would be an opportunity to try my hand at some heavier weathering than I normally do. I know it won't be to everyone's taste, but I am guardedly happy with my decision. Hopefully it looks reasonable. Here is what I did:
After rigging, I mixed a few drops of water with MIG Russian Earth pigment, and liberally applied the slurry to the wheels and lower portion of the undercarriage. While wet, it looked like a muddy disaster, but when after it dried, it took on the more typically matte appearance that pigments deliver. I cleaned it up with a dry brush (and a wet one in a few spots) until I felt things looked suitably dirty. As happens with a wash, more dirt collected along the details, helping to separate them a bit from the sea of green. I repeated the procedure with MIG Black Smoke pigment on the engine firewall.
Next, I filled a bottle cap with AK Interactive Engine Oil (enamel) highly thinned with hardware store mineral spirits, and dipped a ratty old brush into it. With the aircraft sitting on its wheels, I cranked the pressure up on my airbrush (>40 psi), and firing short bursts, knocked the paint mixture off the brush in the direction of the airflow. In a couple of places, I used the air pressure to blow back some of the paint on the aircraft's surface. The aluminum below the engine and the landing gear too the worst of it. There are a few areas that look a bit heavier than I want, so I may go back with a brush and some thinner and clean things up.
Preparing for the upper rigging, I re-worked the wing warping pylon a bit using the 1/32 Gaspatch turnbuckles, and added a pair of eyelets for the rigging wires that stabilize the mast (the reference photos Bertl provided a few pages back were very helpful here):

With a bit of trepidation, I cut off the molded turnbuckle arms, and slit the stub with a razor so that I could insert the turnbuckle eyelet (one side to completion at a time so I had a reference to use to check the angles). I had to twist one of each turnbuckle's eyelets as they come turned 90° from the other end. I used some thick CA to give me a bit of working time, and squeezed the plastic bits around the turnbuckle. They arms seem pretty strong, but I admit I'm a bit nervous about putting too much pressure on them. The eyelets are just twisted wire stuck into holes I drilled.
I should be able to get some, if not all of the upper rigging finished this weekend. Getting close...! (Thankfully, since the deadline for the group build looms.)
Thanks for checking in,
Chris