Proof that He Can?
You are basing your Theory on Pure Assumption.
And you are baseing he cant on pure assumption as well. So your point there doesnt matter.
A better arguement against this theory, would be pointing out that so far none of the bijuu have used any hand signs. And if they cant use hand signs they cant become a bij-jin. The only reason that a bijuu cannot become a jin, is because the bijuu do not have a body. They are simply fragments of the juubis chakra, that manifest in certain forms.
There is no evidence though, that would point out that one cannot seal a ninja inside themselves, o a summon animal like gamabunta. In fact, there is evidence to show it can be done. Hence the juubi. The so6p sealed a LIVING BEING with A BODY inside himself.
The reason that ninjas have not been shown to seal others inside themselves, is because it probably is not practical. Sure the jin may have a slight increase in power, but his/her chakra control may be disrupted depending on the seal. Also maintaining a seal would use up a certain amount of chakra reserves automatically.
so simply the only really effective jins are ones with the bijuu.
(this is a theory based on likely facts)
so simply fuuinjutsu will work on living beings, and the bijuu cannot become jins, regardless of whether they can use hand seals, because they dont have a body to seal it into, as they are just masses of chakra.