does this enable fluid-underground weaving? similar to how underground Y/R/B belts can be woven

No, haven't found a way to do this yet. Looks like it's not possible.

didn't think it was; so it's okay. would have been very impressed though. wish the factorio dev team would make it possible. undergrounds for belts, and pipes both essentially act as a shared inventory with a direction of travel, and underground length built in, so it wouldn't really take MUCH work for them to set it up to be an x,z coordinate of a shared pair instead----making it possible to transfer pretty much all fluids within 1 tile width.

I posted a request for this, so we'll see:)