I also threw in the possibility of the Rocketeer being linked to the MCU when I opened the thread, basically as a throwaway 'What if?'
I've no strong views on this taking place and I'd be quite happy if it's a completely standalone movie. However, I think it could
be done well, even if it doesn't have to be done
. Instead of using Howard Hughes as the inventor, you could have it be invented by Howard Stark (though I suppose given how young and vibrant he was in Captain America, there'd be no reason why he wouldn't be using the rocket pack himself).
I don't think 'It wasn't in the original source material' is any better a reason to exclude it than 'It would be cool if we could do this' is a reason to include it. If it serves the story and helps the movie and they do it well, they could do it. After all, it's not like he's going to be interacting with all the other MCU heroes back in 1930-something. But if it's coming from the MCU stable and is promoted as such, it might help the movie do better at the box office.
Though what I'd really like to see is a shared cinematic universe that features The Rocketeer, The Phantom, The Shadow and Doc Savage...