OK. In case this is of use to others, I’m going to answer my own question:
The Live Send pot is neither linear nor truly logarithmic.
This is of particular importance if you are using it for an equal-power application like panning. Multichannel spatialisation devices like [Max Api SendsXnodes] and [Max Api SendsPan] that control Live Sends via the API and do not address this issue produce a marked drop off in level when sounds are panned in-between speakers in multichannel environment.
I talked to Ableton who say the design is procedural and not given to a mathematical solution, so I did some measurements and produced a lookup table (antipot.aif) to make the necessary corrections.
Here is the .aif file and a small example patch.
I have also applied this technique to a couple of audio-rate M4L devices which I will put up online soon.