National treasures taken from a country and hidden in private collectors galleries are very different to Items of significance e.g. clothing from a rock legend! I would be extremely surprised if Bob Sillerman would break a sweat over minor trinkets that are dotted around the globe... If I recall correctly EPE have sold personal Items belonging to Elvis on the market themselves.
By and large the important Clothing, jewelry and planes are already owned or on loan to EPE. The other sticking point would be that Elvis gave away many items e.g. jumpsuits, clothing & cars etc how would EPE/Sillerman ever lay claim to these items, the burden of proof would be on EPE to demonstrate that any such Items had been stolen, these claims are very difficult to substantiate and would be costly.
I would suggest that this process is a non starter in EPE's case.