so, I can talk to the MxL7004 now
that was an easy one - the I2C bus frequency must run at a much lower speed
but the firmware sucks again; they're writing magic values to undocumented registers, XORing values read from other registers and comparing them to XORed values of other (documented) registers
why on Earth?
and then there's an undocumented register 0xf that can be read only immediately after you read BUCK4 voltage 4 times in a row