I took a piece of never used, OE BMW leather (leftover from a hide I bought in ~2006). I made it into a coffee filter shape and secured it it using a random spring perch, with an inspection nipple poking through the bottom. I then slightly filled it with leatherique rejuvenator:
Today, inspecting the nipple from below, it is visibly damp (and you can feel/smell the leatherique) on the rough side of the leather:
I don't believe there's any explanation for this other than the leatherique going through the top coat and leather, to the far side.
Obviously this is a fairly extreme example, as I left a trapped puddle of leatherique on top of the leather, but IMO it pretty conclusively shows that the top coat is meaningfully permeable.

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