Mark - do you literally think there is no one in aikido today that can replicate Shioda's feats? Or are you speaking broad brush?
I whole heartedly agree if you are speaking generally about the lack of "aiki" among the vast majority of "shihan" out there.
However, as with all generalizations, there are exceptions to the rule. It is my current working hypothesis that Shioda began teaching internal skills to his higher level students a few years before he died to cement his legacy and preseve the organization he built.
How optimum his training methods were is debateable. However, I believe that at least two Yoshinkan shihan can perform most or all of the same aiki demonstrations as Shioda did.
We have to remember that high level jujutsu skills can mimic and look exactly like internal skills. The main difference is to the attacker or uke as they will feel entirely and completely different.
I'm sure there are still people out there who have had hands on time with Shioda. If those people have had hands on time with the shihan you think can replicate the aiki demonstrations, then ask them if those shihan feel like Shioda.
If they say no, you'll know that you're seeing high level jujutsu instead of aiki (again, nothing wrong with high level jujutsu. Great skill to have). If they say some parts do and don't, then you'll know somewhere along the lines, aiki wasn't entirely taught. If they say yes, well, there ya go. A good sign that Shioda passed on what he knew.
With all the aikido people training with Dan, have you? If you've felt these shihan you've mentioned and then trained with Dan, you will have direct first hand experience to make an informed decision about aiki in the Yoshinkan. Like other areas in the Budo world, you won't hear about a lot of things here on Aikiweb, but behind the scenes, it's an interesting turn of events.
I'd also like to ask where you got your information about Shioda getting his aiki skills from Kodo. We all know the stories about the two of them spending time together behind closed doors, but I've been hard pressed to find actual documentation of when they met, how many times, and for how long.
Could Shioda have gained his "aiki" from Kodo? I think it's possible but highly unlikely. More likely he learned certain things from Kodo - perhaps refining his skill or adding to his repertoire.
If you're looking for documentation, I'd suggest the Kodokai. Other than that, I doubt you'll find any in the aikido world. As for aiki ... as I mentioned in another post, I don't think anyone will ever know just how much Shioda learned from Ueshiba or Kodo.
Perhaps Shioda learned a good bit from Ueshiba but was at a point where he couldn't progress and Ueshiba's "spiritual" discourses weren't making sense, so Shioda went to Kodo for a breakthrough?
Maybe Shioda didn't think he learned all that much but when he went to Kodo, a lot of "lights" clicked on from Ueshiba's training sessions?
If nothing else, I would imagine Shioda, Tomiki, Shirata, and some others were chasing the aiki they felt from Takeda. Since that aiki was a rarity, there weren't all that many teachers to turn to and it was a very closely guarded secret. Any avenue to get better was worth approaching. Shioda was very lucky to get to train with both Ueshiba and Kodo.