BTW, having broken 50 myself, I know what it takes.
IMO these guys definitely have the SE to do it, as well as the top-end, the only weakness is in the first 100m from the blocks, around a bend.
You might be surprised to know how much can be gained/lost in the last 100m of a 400m. Let's face it, people who go 50 are not great quarter-milers, and are often those who completely fall apart in the last 100m after running a decent first 200m.
Even running hard (for them) in the first 250m, I don't see these guys falling apart in the home straight--slowing a bit, yes, but rigging, no.
Maintenance of stride length is a big deal, and Rupp would have that. I don't know how tall he is, but he looks to have a decent stride length capability in a 400m-type run.
In the distant past I trained some 400's with guys like that, 400's that did not emphasize the start, but which were fast tempo. I finished ahead of them every time, but after a few I was gassed, and they kept finishing closer...I got the impression that they could keep going at their pace all day long, however.
Tall, thin guys with decent basic speed, a long stride, and good SE can absolutely eat up ground in a 400--look at Rudisha. I know Rupp isn't Rudisha, but I have a hard time believing that guys like him COULDN'T run at least 49.5 for the 400m FAT from the blocks.
As for guys like HG and KB, I would give them faster first 100m segments, because I think they'd be intrinsically faster over a block-start FAT 100m, as well as being able to accelerate better around a curve. What they may lack in stride length, they probably make up in absolutely superior SE. They would also go under 50, IMHO.
And of course KB was massively juiced, so he could do whatever he wanted--plus, I think that his juice would have helped him immensely in a 400m. I would give him 47.5-48.5 if he really tried, FAT from the blocks.