That is an interesting problem, the stock Volvo cars have no idle speed compensation for AC and seem to have no need for it, engine speed only drops maybe 10 rpm when the compressor kicks on under normal conditions. Sounds like yours drops quite a bit more than that, which is not typical. I would say you may have either an issue with the compressor causing excessive drag on the engine when it is engaged, or the injection pump calibration needs to be dialed in further. The governor in the IP tries to maintain a somewhat stable engine speed at idle and does have some ability to respond to load, e.g. if you let the clutch out with your foot off the accelerator the governor adds some fuel to try to keep the engine running. In the Volvo cars, with the Volvo compressor, that governor compensation from the IP is enough to make the effect of the AC compressor on idle speed minimal.
The first question is probably this: what vehicle is the engine in and which compressor is being used, the Volvo one that was with the engine or the one from the vehicle? If it is the latter then it may be a higher capacity compressor that takes more HP to run, and then the kind of idle speed compensation system you are talking about would make sense for the increased load. Since there was no original Volvo system to do this you would probably have to piece together something of your own creation, but it probably would not be hard.
Old Ford/IH 6.9 IDI diesels had a throttle kicker solenoid that engaged below a certain coolant temp to raise the idle for a cold engine. Probably could find similar ones for various old carburetor motors with automatic chokes also. I don't see why you couldn't wire up something like one of those with a relay to activate when the AC clutch engaged, if you could find a simple way to mount it on the IP near the throttle lever.
Here's an example:
https://www.summitracing.com/parts/s...SABEgKD8fD_BwE