Haven't read that Hogan, but it sounds like he might have been inspired by Alfred Bester's The Men Who Murdered Mohammed. Either way, time travelers in their "own" worlds could work.

I subscribe more to the theory (as showin in, e.g., Connie Willis' The Doomsday Book), that time travelers are prepared with authentic-appearing clothing, et al.

Assuming, of course, the extreme unlikelihood of time travelers. (FTL seems orders of magnitude easier, scientifically, than time travel.)