How can I let an isolated Guest Wi-Fi cast to only one Chromecast on my IoT VLAN without exposing the others?

I have a Guest SSID with client isolation enabled.

All three Google Chromecast devices (A, B, and Guest) live on an IoT VLAN.

Goal:
Guest users should be able to cast to Guest Chromecast only and not even see A or B in the cast list.

What I’ve tried:

• Firewall rules already allow Guest → Guest Chromecast IP (casting works once visible)
• Enabling IGMP snooping makes the Guest Chromecast appear for a few hours. I suspect that is because the client device I was using to test was on my home VLAN prior to testing different firewall rules on the guest VLAN, so the Chromecast was "cached" and thus visible.
• Enabling mDNS on the Guest network makes all Chromecasts visible, but only the Guest one is actually reachable due to firewall rules

The problem is discovery — I can’t make the Guest Chromecast reliably appear without also exposing the others. Does anyone have any ideas?