Make so you can turn on option to make spawner work even if no player nearby.

This would require the chunk to be loaded which is a problem outside the scope of this mod.

This would require the chunk to be loaded.

That's not the point. I'm asking to make this mod force mobs to spawn. Chunkloading is some other's mod problem.
So far there are not that many mod provide ability to force spawn spawners (Draconic Evolution, Thaumcraft, i think. Soul Stones) but most of this mods add many extra blocks and items. Also, there are quite alot of mods that can chunkload.
Your mod can control behavior of spawners, change frequency and amount of mobs to spawn from simple spawner. Only one feature it's missing is forced mob spawn.

So alter the vanilla spawning rules, which means you could spawn any mob anywhere under any conditions?

Not exactly, but as addition.

You know mod Draconic Evolution? There is spawners that can spawn mobs you insert soul in. This spawners can accept 2 crucial upgrades.
Golden Apple upgrade make so mob can spawn in any condition, even mid air or underwater. Any mob.
Nether Star upgrade make so mob spawn even if no player around, as far chunk is loaded, mobs will spawn.

I can look into it! But I wont promise anything :)