we're saying that the design was developed in the early years of the Galactic Empire, designed for the express purpose of allowing Jedi to conceal their lightsabers from notice by imperial forces. It was designed by a Jedi padawan who had no traditional training, leaving him to invent his own solutions.

While it is technically possible for someone to have thought of a similar idea in the past, the design in [i]Rebels[/i] was an inferior blaster attached to a lightsaber, in an era where weapons in general had far more advanced designs than in this era.
Among lightsaber designs, 99.99% of users used either standard hilt, curved hilt or a saberstaff. Variant hilts that were not one of those three were extremely rare occurrences. Even Jedi (or Sith) who used both lightsabers and ranged weapons of some form were exceedingly rare. Jedi (or Sith) using non-standard lightsaber with a built-in blaster of some sort would be even rarer than that.
Such weapon would typically have very few shots or low effective output, increased weight of the lightsaber significantly and reduce its versatility significantly.

Thus the odds of anyone having such a specific variant of their primary weapon in this era are astronomical. Anyone applying with such would have extremely good reasoning for it to be approved.