Didn't nagato absorb Bee's chakra not only the Kyuubi cloak when he touched him ...Either way it proves that he can absorb chakra without him initiating the contactRinnegan=/=Sharingan. It doens't change the fact that the Kamui barrier originates at Obito's eye so the chakra is coming from his eye. Period. Chakra is used to activate the barrier but the barrier itself is not made of chakra. Period. So Kamui isn't getting absorb. Period. Obito's Kamui being absorbed is probably one of the stupidest things I have ever heard on the base. It makes zero sense.
You are going need scans of the field of absorption absorbing chakra directly from the person, and you are going to need scans of Preta being able to absorb chakra from someone, without the user of the technique touching the target first. Obito touching Nagato doesn't equal Nagato touching Obito, get that thought out of your head cause it makes no sense.
Although ya i just rememebered they get absorbed into the eye anyways lml, i guess he can win here but only due to izanagi ...i think
Also you only said Dojutsu so i said the rinnegan technique