I'm not sure how in-line this feature would be with what you want Replay Mod to be, nor how complicated this would be in implement, but it would be a great addition if it was possible to add your own light source, and customize it's colour, position, range, etc. Sure would be helpful for editors like myself.
Imo this doesn't really belong in the ReplayMod because it might as well be a separate mod. Also, adding something like custom lighting to Minecraft is anything but trivial as it might require re-writing half of Minecraft's rendering engine for it to work properly.
I am however currently tinkering with something along the lines of this (exporting parts of a replay to a blender file). Such a feature would allow adding custom lighting (and pretty much custom anything) to the scene in blender. It still has gigantic issues that need fixing though (e.g. enormous file sizes, unbearable ones when particles are included, so basically no particles for now; no enchantment glow; no beacon beam).