I have been trying to find a filter that uses the progress reporting so that I text that code in gmic-sharp.
The G’MIC Reference lists the progress reporting commands, but it does not say what other commands make use of it.
Most commands does not utilize them. You can try searching for them using updatexxx.gmic in appdata/gmic.
Thanks.
The good news is that G’MIC is calling the progress callback, the bad news is that I now have an access violation crash to investigate. 
Typically, the “Shapeism” filter (command fx_shapeism) could be tested. It uses the progress command.