After a series of compromises, we got this working by doing the following: 

Create a flow that runs when a Request is created, then look up an SCTASK where the REQ number is the same as the triggered REQ, and the SCTASK is an order guide SCTASK.

If a record is not found, then the REQ was not created from an order guide and the rest of the flow fails (correctly). If a record is found, the entire REQ was created from an order guide. The flow then generates a link to the Portal Request Summary page for the REQ and sends that in an email to the Requested For user. From there, the end user can see all items ordered on that REQ.

Generating the link to the Portal Request Summary page was more complicated than expected. I used a slightly modified version of this link generator by Kieran Ason to get it done. 

A downside of this method is that this flow runs on every REQ created, and most of them don't have anything to do with an order guide, so the flow execution log is filled with lots of failure logs. Those failures are "accurate", but still not fun to see. I'm sure there's more elegant ways to accomplish this need, but hey, it works.

Flow screenshot below for those curious.