[derefr]

Why not use Talescale / ZeroTier / etc to get those local boxes connected together into a single virtual overlay network with that gateway (which, from the client site's perspective, is basically just "install app, paste pairing token", and is zero maintenance from then on); and then make your web iframe "gateway" discover these devices that are now on its network, and act as an actual request-proxying gateway for them?

This is similar (though simpler!) to how e.g. Plex's "watch from anywhere" cloud integration works. Your local plex-server install starts a (plaintext HTTP, Bearer-auth) HTTP server on some uPnP port on your local network; derives a public URL for that server via TURN/STUN; connects to Plex's cloud service, and registers that URL to your Plex account. Then, when you visit https://app.plex.tv/ from any device, the plex webapp talks to the backend of their cloud service; and that cloud backend takes your HTTPS request payload, and makes equivalent plaintext-HTTP requests to the registered Plex server on your local network.

[noduerme]

TBH, this system has been in production since 2016, and the stores essentially can't operate with any downtime... I dispatch updates that are auto-downloaded, and when one of them misfires and I have to spend an hour on remote access, that store's business comes to a halt causing a huge disruption as they transition to paper notes and whiteboards. (The software lets employees track and take ownership of the checkin/checkout and movement of dogs around 30k sqft petcare facilities, each with hundreds of dogs per day... so just the amount of barking when I'm on the phone with these places makes it hilariously difficult to guide people through tech support).

I looked a little bit at Tailscale but assumed it would be too complex for the store employees to manage and would require a lot of hands-on maintenance, and a store-by-store hand holding process which requires scheduling each of them weeks in advance. As it is, the system is secure (local wifi only accessible to employees) and runs even if the store's internet connection goes fully offline... so... I think I'll take your tip and look at it again... the plex model is essentially what I was going for... but I'd be reticent to add any other layers of complexity or anything that could cause downtime.