Waiting for a 500 version of the J engine could take rather long. If it ever happens at all…
You are thinking that RE started with a blank sheet of paper, drew an OHC, added a balance shaft, and reworked just about every other aspect of their existing engine design, but kept the Bore x Stroke at 72.0 x 85.8 mm, (.84 under-square) just for tradition?
And calling that RENA guy a liar to boot?
Did you notice that a 86.0 x 85.8 (square) - bore x stroke gives a 498cc capacity? Now, it's true that when RE designed the Himalayan's 411, they gave it 78.0 x 86.0mm, (.91 under-square) but that's a machine that is designed to handle off road riding, where the 350s are all road machines. And RE isn't married exclusively to long stroke designs, the 650s are 78 x 67.8 mm (1.15 over-square).
Now that the 350 OHC "J"s are fitted with all the systems required to meet current standards, if the engineers did their job like I think they did, a 500cc version should be as straightforward as fitting new jugs and heads. It would be interesting to compare the 350 OHC crank to one from a 500 UCE, or even a 411 Himi engine.
We didn't get many 350 UCEs or IBs imported to the USA, I've yet to see a single example in person, but I wonder if somebody pulled off the "350" badge and stuck on a "500", anyone would be able to tell the difference? My point is, the 500 OHC prototypes might already be out there, and RE wouldn't need to shroud them in "camo", they could just build them on shiny new Meteors and Classics.
The RENA guy couldn't tell me "when", and admittedly "if" could change. But, the 500 UCEs were killed more by price increment from the 350s than by lack of desire to own them. The IB 500s sold much better, compared to their corresponding 350 version, than the UCEs, because they weren't saddled with a new, untrusted, and expensive FI system.