Going into virtually any aikido dojo on the planet would be fine for getting in good aiki practice as nage... no one would be the wiser when you apply aiki to your waza. They'd just think that your aikido is fabulous, perhaps mysteriously so. On the other hand, I'm not sure that taking lots and lots of hard ukemi in such a setting would serve as a necessary prerequisite to learning aiki (there are other, less bitter-eating ways); but, taking ukemi of any kind, copiously, from someone who has aiki could offer some opportunities. In taking ukemi for someone who is aiki adept, an intuitively astute student can "steal" ... by feel ... what nage is doing internally. The more ukemi, the more opportunities to cop that feel, so to speak, and make it one's own.
Taking ukemi from someone with these skills is VERY different in a fundamental way. I remember being shocked when I first encountered Yamaguchi Sensei about twenty-five years ago, and noticed that he just felt different from anyone else I had ever practised with. I just couldn't feel what he was doing - whatever I did, I found myself falling over without understanding what was going on at all. As well as that, receiving technique from him was simply exhausting in a distinct way from the familiar fatigue from the rapid succession of up-down-up-down...
In those days we just talked about him as a special case - Yamaguchi just HAD those skills, and we were a different kind of mortal, with no chance of picking them up. Since then I have had this experience from more people: mainly Kanetsuka Sensei, Yamashima Sensei, Ikeda Sensei, Endo Sensei, and I am starting to get a clue of what is happening, even if these teachers aren't all necessarily very good at explaining what they are doing, nor how we might learn to be able to copy them. I have had several rather seminal experiences from Kanetsuka Sensei (the only one I practise with regularly) when I have quite definitely felt something that was extraordinary, but which I could start to work on. I think this is the real meaning of direct one-to-one transmission, since these things are very hard to get across in a class teaching situation.