I have another idea though this is kinda niche again.
If you look at Intel NUC Compute Elements, they can be plugged in into the PCIe slot and act as a secondary PC for video capture/stream processing.
Maybe we can reuse mainboards to work like that too with a 3D printed PCIe adaptor and maybe a PCIe 8-pin power connector (to the battery or as a special expansion card) to power it up. When the main PC boots into Windows, it will be recognized as an additional card that can be selected for encoding etc.