Well explicated, Jim.
The connection is fairly simple-- the dominant Islamic culture in that region for centuries was the Ottoman. As I discussed in
On the Persian Shamshir (2001, available here:
http://www.vikingsword.com/vb/showthread.php?t=20604) , pictograms gradually overtook inscriptions in importance on blades in Islamic cultures. This happened for a variety of reasons: in the case of Persian blades, I showed that lion marks evolved in recognition of an Assad Allah brand (which had more traditionally been inscribed in Arabic characters,) intended for a largely illiterate clientele. With Sudanese blades, however, the adoption (and adaptation) of the tugra to local owners was a simple matter of association with Ottoman authority and thus, power. Incidentally, on the topic of literacy, research would probably show that a large percentage of the Mahdi's followers were literate, certainly among his chiefs/ officers.