You can't create new entity types (also called prototypes), since those are hard coded right now.
However, you do have several options you can change to make it act like you want to.

In your example, a centrifuge acts just like a chemical plant, but has different recipes. Then copy the entity ...

oncrafteditem - called when a assembly machine / furnace creates an entity. returns item/entity
For that you could call the Entity.getoutputinventory on the furnace/assembly to check for updates. If an item appears in the output inventory without one having been taken, then it has crafted one item ...

I'm not sure if you know about this, but you can use the game.entityprototypes table to access properties of entity prototypes from control.lua.
That is a good one, I did not know about it, but as FreeER said, it doesn't give you access to all properties.

it's a desired feature that's simply not ...

I would think it would make modding very much easier if there was a method to get a hold of the properties set in files like entity.lua from control code, like control.lua.

In the past few days I've been doing things where I wish I could just write a line like
area = event.createdentity ...

Thank you, I think simple-entity might be a good option, since it doesn't seem to have any other functionality code around it and thus will probably be the most future-proof option.

I ran into a problem when I wanted to make an entity that doesn't share nodes with any vanilla entity type, so I thought I'd create my own. Unfortunately, it throws the error that the Loaders don't know how to handle that. I really don't want to hack an inserter or assembly machine or anything like ...

Is there any specific reason to have a brand new tech unlock for it? As I see it, from a game story standpoint, it should really be the same process as filling crude oil barrels, which you get from Liquid Handling (1?).

Great mod though, I've always found it odd that only crude oil can be barrelled.