I wanted to post a belated update here, in case it helps someone else or could trigger additional discussion or action. Per Jon G, I emailed CiviDesk and got a prompt response:
We have developed multisite support for events in CiviCRM, and it has
been in production for several years now with one of our largest
customers. The code is public, but not documented or guaranteed and
delivered as-is, without any support from us.... It is available in
contains a lot of patches in addition to just the multisite changes.
We will be migrating this customer to CiviCRM 5.7 ESR in H1 2019, and
port all to this newer version then.
CiviDesk further clarified that the customization does not currently include 'packaging' for this feature (i.e. setup screens, documentation, test suite, ...). It cannot become part of the official CiviCRM release without these. If there were interest and funding (via a "Make it happen" project or similar), they could move forward on this.
I had hoped to dig into the code to explore further, but haven't yet because of limited time and other priorities. For the short term, we're using a WordPress-only multisite compatible events management solution. For the longer term, a solution integrated with CiviCRM would be preferable. With guidance from more experienced Civi developers, I'd be willing to pitch in.