Can I share a managed AP with another network?

Sorry for the horrible title.

I set up AV systems. We always have a router in front of our system, pick up WAN from the customer's network. This can be direct from the ISP modem or from behind their firewall. Normally, we add our own wireless APs behind our router but for a job that I didn't spec, we don't have APs.

I need to get an iPad on our network for local control of some devices.

Assuming the client has managed APs, will this set up below work?

I'm thinking their IT Admin creates an SSID for AV. That network would untag to an AV VLAN. On their switch, they untag an Ethernet port to the same VLAN. Then, I connect that port on their switch to a LAN port on my switch. In theory, this would have our router assign WLAN clients IPs.

Thoughts?