MIDI assign while Ableton is ReWire Slave?

[Heat Death]

Hi, l know how to assign MIDI controls while Ableton is on its own.

However, l cannot seem to find a way to assign MIDI controls to Ableton while Ableton is a ReWire Slave (FYI, the ReWire host is Reaper).

I've tried assigning the controls while Ableton is alone, and then loading the ReWire setup, and loading into Ableton the saved Ableton project which has the MIDI controls already mapped, but to no avail - Ableton ignores all MIDI assigns when it is in ReWire Slave mode.

Or does it? Is there a way to do it?