You didn't say how the 66u is connected to the 87u.
The 66U is connected to the 87U by connecting the WAN port of the 66U to one of the LAN ports of the 87U. The 87U gives the 66U an IP address and takes care of all the DHCP duties of the devices connected to the 66U.
I may have made some progress since my last post (I decided to give it one more shot).
Here's what I did. On the 87U:
LAN -> IPTV: Enable multicast routing (IGMP Proxy) : Enabled
LAN -> IPTV: Enable efficient multicast forwarding (IGMP Snooping): Enabled
Wireless -> Professional -> Band 2.4GHz -> Enable IGMP Snooping: Enabled
Wireless -> Professional -> Band 2.4GHz -> Multicast Rate(Mbps): OFDM 12
Wireless -> Professional -> Band 5GHz -> Enable IGMP Snooping: Enabled
... and a reboot.
On the 66U:
Wireless -> Professional -> Band 2.4GHz -> Enable IGMP Snooping: Enabled
Wireless -> Professional -> Band 2.4GHz -> Multicast Rate((Mbps): OFDM 12
Wireless -> Professional -> Band 5GHz -> Enable IGMP Snooping: Enabled
Wireless -> Professional -> Band 5GHz -> Multicast Rate(Mbps): OFDM 12
... and a reboot.
Now when my iPhone or Mac is connected to the wireless network (2.4 or 5) on the AP (i.e., the 66U), the AirPlay icon appears when it did not before. I can ping/telnet/etc, using the .local hostname (i.e., Couch-potato.local) now, and I can connect via AirPlay.
Now to see if it lasts, because other people have complained that even if it appears to be working, it often just stops after a period of time.