In addition to trying this (or the part that doesn’t involve the egress priority I’m defining myself through an explicit VLAN that includes a DEI bit that’s being ignored), I’ve also managed to stumble across the way to map a given priority to a given TC, but regardless of the “lossless” quality of TC 1 (mapped to priority 4) and 2 (mapped to priority 5), these mappings seem to have have no more effect in determining what is drop-eligible than setting DEI to false for priorities 4 and 5.
In addition, having to run both mlnx_qos on the server as well as configuring through the CLI complicates things to the point where it seems redundant or even contradictory, and in any case, as incomprehensible as reading uncommented code.