One of the Remote Multicast Setup specification features is to create a multicast channel and the other associated session.
Nothing prevents to create a multicast channel and to not uses it.
In this case we are just interested in having the end-device to switch to Class C at a given time with given parameters for a given period which is known from both sides of the network.
The multicast downlinks are the same as unicast the only difference being the DevAddr + Keys being used.
I did not try this my self however I suppose that ChirpSTack provides means to trnasmit unicast or multicast downlinks upon user request.
Another possibility would be to use LoRaWAN 1.1.x version which defines a MAC command which indicates to the network server when the end-device has switched to ClassC.
Please look at chapter “18.1 Device Mode (DeviceModeInd, DeviceModeConf)” of LoRaWAN 1.1 specification.