Well, we know it requires a Lerge amount of chakra to "perfect". The fact that it is unperfected means that it's either only been discussed and not tried because they know the possible side effects (maybe fatal), or that the jutsu has been attempted but naruto lost control when he tried to tap into the fox's chakra.
This jutsu also must be something that Minato used/tryed to use and Jaraiya either also tried to use or has vast knowledge of (you cannot teach a jutsu without knowing how it works, amirite?)
A sealing rasengan seems to be an oxymoron to me. seals seems to be mostly defensive/supplemental types, mostly to lock something away. but rasengan is an attack type. so I don't see it happening, but it would be awesome if it did xd
Lets brainstorm here and wonder what would a sealing rasengan seal?