Nagato transmits Chakra through the Paths and kneads Chakra for them to utilize jutsu. There's no reason why he can't apply some form of sensing for either of the paths.
Rinnegan also had Chakra Vision as it could detect the barrier surrounding Konoha.
Perceiving chakra build-up is simply using some form of Dojutsu/Sensing that can detect Chakra being built in within a part of the body and react according to that. Deva Path displayed that he could indeed perceive chakra build-up as he noticed Tsunade storing chakra within her feet.
Baseless assumption is baseless. "No reason not to be able to" is not a viable premise. Never been done, never will be, yet it should've at a few occasions. Don't you think they would've tried to sense Naruto (who would've been hiding) from the beginning instead of trying to get intel by whatever means? Yea..
When did I ever say it didn't lmao? It sensed the chakra surrounding Tsunade's feat too, doesn't mean it can sense
internal chakra build up like the Byakugan does (refer to Neji vs Kidomaru). Never been shown, never will be.
He noticed the chakra build-up outside of her feet. You place the chakra
beneath your feet, not within them.