Okay, thanks for that, I won't pursue that direction further.
But meanwhile, there was this post in MC18 Forum:
MadVR Profile Groups
I started using Profile Groups to enable different scaling algorithms based on a condition. For example, my APU can do Jinc3 chroma upscaling for 1080p 24hz content, but it drops frames for 29.97hz interlaced. I have setup 2 groups: one for 1080i with Bicubic chroma upscaling and 1080p with Jinc upscaling and created a condition under the profile group: if (deintFps > 24) "1080i" else "1080p" which will select the appropriate chroma setting based on the source.
To create a group, select the scaling algorithms folder -> create profile group and choose what scaling you wish to create a group for (1 of each type is possible). Create each profile under the newly created group and name each accordingly. Use the name of each in the condition expression as shown above.
which would solve the problem perfectly.
In madVR 0.86.9 which is in MC19 build 108, I do not have a "create profile group". That seems to have been added in 0.87.0, which also adds "* settings can now be edited without madVR running (only on local PC)" which would make what I am doing at the moment somewhat easier.
So, a request for updating MC19 to 0.87.4 (which includes some fixes), thanks !