The HY/RD calipers have a rubber bladder under the cap, and work horribly if there's any air in them, so you should be able to turn them upside down without issue. I would actually say that the angle my HY/RD sits on my fork, it's upside down already.
A typical hydraulic braking system is "sealed" as far as something that presses metal pistons through rubber seals can be sealed. If it were perfectly sealed, the fluid would never get fouled, but the pistons also wouldn't be able to move.
Sealed comes in varying degrees.
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