Would it be possible to create some hooks / areas that extra mods can use to modify the rendering, both in recording and the editor?

e.g. I foresee that a companion mod that works alongside ReplayMod could create additional titlecards, create fonts rendered in world-coordinates, or highlight / fade out a region of blocks, or trigger litematica to show a layer rendered.

The litematica part is not possible, as it's a client-side mod, the server does not send the data of the schematic so ReplayMod can't catch it

Unless a companion mod recorded that information for Replay Mod, using an API