Daylight harvesting and variable targets

I'm assuming I'm going to need to use some logic to achieve this hence posting it here.

I have an application where a reasonably large area is going to be set up with daylight harvesting, targeting 500 Lux at desk height; this will be achieved by controlling the lights to achieve that level measured using a calibrated sensor, recording the lux level reported by the C-Bus PEIRL sensor, and setting that as its target. There will be several PEIRLs across the area, several dimmable circuits (via DALI), with the whole area being on together (at the same Lux level) or off (ie consistent across the area when it is occupied anywhere, not just under the sensor that triggered). So far so good (he says pretending that bit is easy...)

However, that 500Lux is actually intended to be the "maximum" level, and they want to be able to ramp up/down using a C-Bus switch (actually a retractable switch and Bus Coupler). So ramped to fully on we have a target of 500 Lux, ramped to 50% a target of 250Lux, ramped all the way off a target of 0 Lux, etc. Those targets being desk level targets so related to but not the same as the sensor targets.

First question: Is this even remotely possible? Second question if the first isn't "no": how to achieve it?

"Logically" I think something along the following lines would get somewhere close: When user presses the switch, disable daylight harvesting and ramp the lights directly up/down. Once settled at the new level, read the new lux level at the sensor and set that as the new target to maintain, and re-enable the daylight harvesting. Although how to do that reliably across (say) a dozen sensors, and how to ensure that they can't ramp up above 500 Lux total, I have no idea. Also, ramping down until the lights are off in daylight will set a target that the lights will have to come on to achieve when it gets dark outside. So the whole thing sounds implausible and that's what I think I need to tell the client unless someone can tell me otherwise?