After weighing all the feedback I've decided to stick with using the Dornier Do-J "Wal" as the Dyer expedition aircraft.
There's certainly a case to be made for the Merkur, but in the end I found myself agreeing with those that sided with the Wal. Frankly, it just looks better. The Merkur is a fine little aircraft, but it suffers from looking like all the other single-engined planes of the classic era. The Wal is nothing if not distinctive.
Here's the basic color logo I'll be using for the next run of swag. There are some relatively trivial composition and scale changes, but the biggest difference will be in the background colors. The dark blue of the original design will be lightened a few shades to provide more contrast with the plane in the embroidered versions. The dark grey of the outer text ring is now the same red as the Miskatonic University logo, giving the overall design a bit more pop.
I'll finish up the black and white version, and a stencil template for marking gear and equipment crates, over the next few days.