Just an FYI since there is a lot of knowledge in here that is getting mixed up with with other things.
Bluetooth Channels: Technically, a BT device should have 8 Channels, 1 for itself and 7 available for other connections. Channels allow multiple devices to connect to the same device. It in no way what so ever has any effect on other BT devices and their available channels.
A Wii has atleast 5 channels enabled, itself and 4 for Wiimotes. It is unknown if the other 3 channels are usable on the Wii.
To clearify for the OP's question:
You should have no problem with multiple Wii's in proximity of each other and Wiimotes.
When a BT device connects to another BT device, it does so linking to its MAC address. Every BT device has one, assuming its regulated properly and to code, there should not be any MAC address duplicates between Wiimotes.
There should be no issue connecting Wiimotes to the different Wii's as long as it is initially done in order (power on 1 machine, connect 4 Wiimotes, power on machine 2, etc).
You will probably run into problems if someone powers off more then 1 Wii at a time. Also if one Wii does not have 4 Wiimotes, and another machine is powered off, those Wiimotes might possibly attempt to connect to the Wii with an available connections.
In the end, if you can get together 5 other Wii's in addition to your own, surely you can get a 2nd Wii to atleast test what i stated before the event.