I believe the answer to your question is no, he could not.
It took him hundreds of tries under artificially perfect conditions. There is zero chance he is going to walk into a strange room for an exhibition and run 500+ in front of spectators, on any size table, on a single attempt.
Nonsense. There is always a non-zero chance JUST LIKE Willie walking into an exhibition and running 500+ on a single attempt. We don't know if John tried hundreds of times but the fact is that Willie played hundreds of straight pool matches, many in round robin tournaments and almost every night for like 6 months against Greenleaf on the road which he credited with teaching him a TON. John had no such experience playing 14.1 continuously for decades against the world's elite players so he had to create it.
Maybe Willie didn't even know if he could run 500+ and if he had never been close then FOR SURE you would have said that there is nada, zip, zilch, no chance that he could walk into a room and run 500 on the first try.
You shouldn't confuse capability with probability. John has shown that he is CAPABLE of running 600+. So that means he COULD repeat it. The likelihood of him doing is low because the likelihood of ANYONE being able to rattle off that type of run at any given time is slim even for the world's greatest players due to the difficulty of the feat and the fortune needed to get makeable shots over the course of dozens of racks.
In fact, the fact that it has been claimed that
Willie gave hundreds, perhaps thousands of exhibitions all over the country, in different rooms and on different equipment, with different conditions, 300 days out of every year, for years.
BUT, he only gave *one* exhibition during which he attempted a high run ;-)
Means that, if true that Willie only did one exhibition where he attempted a high run, that him running 526, on that one day, was likely far more probable than we know - OR - it was a complete fluke, right time, right table, right mindset. But in general IF he wasn't trying to run more than needed for these exhibitions which seems likely because it was reported that he couldn't stand doing them, then it means that he had the capability all along to post runs in the hundreds but just didn't bother to try.
John on the other hand has actively trained to run big numbers so he should be even more capable of putting up a big run at any given time.
One thing I would think would be a way higher probability would be that John could walk in cold and on the first attempt run twice as many balls as average players with hundred ball runs.