RE: BSIP-020 Draft - "Introducing profit-sharing/dividends to Bitshares (UIA only)" - Input would be massively appreciated!

Can you clarify the significance of the fractional reserve on the reason for an EBA not being eligible?

Sure, so the guarantee/promise that Exchange Backed Assets (EBA - such as OPEN.GRC) provide the Bitshares DEX is that behind each individual OPEN.GRC token, there is a real Gridcoin token that an user has deposited onto the BTS DEX via the Openledger gateway.

If OpenLedger was to issue additional OPEN.GRC tokens without real backing Gridcoin, then they would be operating a fractional reserve (OPEN.GRC would decrease in value less than 1 GRC per token). Openledger does not operate a fractional reserve, where as centralized exchanges potentially do.

How OPEN.GRC could be eligible for dividends:

Openledger could place an additional market fee (on top of their current fees) and redistribute a portion of these fees onto OPEN.GRC holders (much like they do for their OBITs holders).
A 3rd party UIA is sharedropped against OPEN.GRC holders.
Openledger buys real gridcoins, deposits them onto the BTS DEX as OPEN.GRC then sharedrops them against OPEN.GRC holders (unlikely).

Also, how might the code distinguish between an EBA versus another UIA?

The code wouldn't, it's more of a theoretical/political ineligibility than a technical limitation.