@billmerryman, can you clarify about what you mean by the DXL HAT “powering the Pi”?
When I looked more closely at the documentation of the RPi Zero DXL HAT, is is using DXL Protocol 1, so AX series is OK, but not XL series which is on Protocol 2.
Back in 2021, ROBOTIS did have a working prototype of a RPi HAT for DXLs (for beta testers only), and using C/C++ programming I got it to run 9-10 times faster than the U2D2 out the USB 3 port (and that was on RPi4B).
When the RPi5 came out in 2023, I did test it on RPi5-8GB, and it worked fine too, but its UART port is remapped very differently with the RPi5. Please see this post:
Not sure why so far ROBOTIS has not released the DXL-HAT to the general users 
Regarding the OpenCM-904, ROBOTIS retired it a couple of years ago, currently favoring the use of the OpenRB-150 which is an Arduino board based on the MKR ZERO. The RB-150 can be set up to act like a U2D2 but at max 1 Mbps. Recently ROBOTIS used two RB-150s to communicate between a Ubuntu PC (which can be an RPi 5) and two robotics arms in their latest Physical AI kit called OMX (see links below).
So far, the DXL communications trend seems to be going with USB devices for ROBOTIS - that is my own observation anyway. Come to think of it, do you remember the USB2Dynamixel module (also retired) from way back when, with the original BIOLOID and AX-12s? In some way the RB-150 is an upgraded and programmable USB2Dynamixel. In theory, the OpenRB-150 can be programmed to pass-thru DXL Packets while handling the RC-100 communication packets for other work, but so far I do not know if anyone has written such an Arduino code to perform such features at the same time for the RB-150.
At present, only the NEW RB-86/88 controllers from ROBOTIS have firmware that can do this feat, please see this post: