Demetrio - I have two thoughts on the street clothes, based on what I knew of him.
1. If he was wearing a uniform, he'd be proclaiming he was "fighting fit." In wearing street clothes, he insulated himself from such claims - like a boxing trainer who goes ringside in street clothes, as opposed to the boxer in trunks.
2. Like a lot of shitamachi guys I knew, Kuroiwa sensei had an extravagant (almost arrogant) humility. As a boxer, one side of him found the pretentiousness of so much of the martial arts world to be amusing. Wearing street clothes was a commentary, of sorts.
On one of the several occasions he taught at the original Bond Street Dojo, I had the opportunity to hear him respond to a question about how the "street clothes demo" memorialized in that Aikido Journal video came about. His answer was that a student who was to have been in the demo was charged with bringing both two uniforms: his own and Kuroiwa Sensei's as well....and he got lost in the Tokyo subway on his way to the venue. Not only Kuroiwa do that demo in street clothes, he did it with someone other than his intended uke. (Rumor has it that you're not unfamiliar with doing such things! Apple|Tree)
I hasten to add that I don't feel this negates your remarks in any way. The attitudes you identify were so much a part of him that the situation simply gave him a socially appropriate occasion to display them publicly and impeccably; after all, he had promised to demonstrate that day, and regrettable though it might be that he didn't have a keikogi and hakama, honor demanded that he fulfill his promise. Who could quarrel with that?