Thanks for your assistance. Robertson's other reference is from page 419, Robert L Dabney's Life and campaigns of Lieut.-Gen. Thomas J. Jackson, (Stonewall Jackson), 1866.
This is a Google book; on pp 419-20 Dabney also makes no mention of Capt Mason or black pioneers.

Checking Robertson again, Capt Mason and his black pioneers are mentioned on pages 387 and 487. Evidently Mason built bridges by removing wagon beds from axles and laying boards lengthwise across the beds. Dabney mentions that this method didn't work well at Port Republic, several soldiers falling from the makeshift wagon bridge in midstream. One the front axle of one wagon was placed against the back axle of another in such a way that a substantial drop existed from one to the other. This created problems in the midnight darkness.

Based on pages 387 and 487, Robertson's source for Capt Mason and his black pioneers are unpublished papers of Jed Hotchkiss. Indications are that Capt Mason did not have a commission in the Confederate army ("acting quartermaster") and the number of black pioneers was small (a "little band"). Robertson suggests that these men were present during the Seven Days Battles; the Jones quote from my earlier post tells us that Mason had no men immediately available.

The contract for this group of men expired on July 1, 1862. However, even if Capt Mason collected another "little band" and went to Maryland, the small number of slaves shouldn't have made much an an impression on observers.

The Confederate government should have made better use of men like Capt Mason and his slave laborers, but that's just my opinion. It might be worth noting that by June 1862 Bragg's army in Mississippi included several organized pioneer companies. Enlisted men for a company came from regiments assigned to one brigade; officers appear to have been specifically selected by brigade commanders.