The WinXP+PCem combo just wouldn't be viable at all here (and really this applies for most modern-ish PC games):
- This is an emulator. An emulator for a really old PC. It's not like libTAS being an "emulator" simulating an extremely powerful machine (with its ability to just lie about time). Rather, you will be dealing with running the game at however fast that really old PC can run it (which for Hollow Knight sounds like it would not be able to maintain full speed at all, so you can imagine that, assuming you were running PCem itself at full speed, you'd still be running the actual game at 5-10 fps or whatever, and that's going to be reflected in an encode.
- Hollow Knight, looking at its Steam page, already has a Windows 7 requirement, which cuts off support with the WinXP+PCem combo.
- Hollow Knight also, on it's steam page, seems to have a D3D10 requirement. This also cuts off WinXP, which only supported at most D3D9. Even then, the best GPU that PCem supports (3DFX Voodoo 3 3000) only supports up to D3D6.
- There is also a likely chance that even the 32 bit version (which apparently exists for HK?) wouldn't work, as it's highly probable it is built assuming SSE/2 support. With more modern compilers (e.g. more modern MSVC / gcc) the developer needs to add additional special flags to actually make a compiler spit out something not making these assumptions.
- 2 could in theory be worked around by using some modern Linux distro. But then you are swapping out 3's D3D10 requirement with some OpenGL requirement. The best GPU that PCem supports only supports up to OpenGL 1.1. For context, this predates support for shaders (also similar case with D3D6; D3D8 was the first D3D to support shaders). This itself is going to eliminate practically any modern game that uses the GPU as they are going to be using shaders, not the ancient fixed function pipeline ancient GPUs used (which is even possibly outright unavailable for newer GPUs, so even more reason for modern games to use the more modern APIs which that Voodoo card fundementally cannot support).
- 5 could in theory be worked around by forcing software rendering. Which just makes 1's issue get turned up to an instance level. Keep in mind that software rendering is going to be happening on that emulated CPU. An emulated CPU that only has 1 core (and no hyperthreading magic), and is only running at 450MHz, and predates even the basic SSE extensions that have been present for over 20 years.
Simply be, PCem is very much not a magic bullet once we talk about any modern-ish PC game. Especially not Unity games (along with most modern-ish PC games), which would all use shaders, and so fundementally would not work here.