Love my old i1D3.

But measuring my Jvc projector with it, off the screen, can be frustratingly slow and sometimes impossible in the lowest ire levels.

The usual workaround is to measure D65 at 100 ire off the screen, and use that to correct all the other measurements which are then made with then i1D3 pointed at the projector (with the diffuser one).

Is such a technique or workflow documented for ColourSpace?

Just use Probe Matching.
The Reference will be off the screen, and the Active off the lens with the diffuser.
You need to match all four RGBW colours.

Steve