Custom Render Layers on Vanilla Entities

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The renderers for entities, apart from rendering themselves, also render a list of extra layers. For the player renderer, that would be the elytra, the cape, armor, arrows, held item, DeadMau5's ears, etc.

Say I want to add one myself, for, say, rendering a cupcake on the player's head.

Is this the correct approach to rendering extra stuff around vanilla entities? If not, which one is? If yes, how exactly does one add extra layers, when one doesn't have access to the RenderPlayer constructor, where they are added?

There are Post and Pre render events, which seem viable, but do they have access to parameters that the LayerRenderer#doRenderLayer() method has (stuff like limbSwingAmount, etc)?

RenderLivingBase#doRender(), which calls doRenderLayer() for all the layer, calculates all these parameters from the entity's fields. Thus one could calculate them for oneself in the event, but that seems wasteful since the values are already being calculated every tick.