ESI Block fails to insert ESI URL onto page

Activating one of my context-placed blocks for ESI whisks it right off the page. I followed this to an implementation of esi_context_context_load_alter() which I thought had a minor chance of causing friction, so I tried to smooth out some rough edges. Moving on, I found the problem appears to be the block system not knowing about the ESI block that is being used as a substitution.

I've created hook_block_info() as a simple relay of a statically cached registry populated by esi_block_register_block(). It works for my context use case as patched, though would not be ideal for sites using lots of ESI-enabled blocks.

Setting to needs work because the patch does not account for the needs of core block or panels, though in theory that should be an easy addition if this direction is suitable.