Connect bus wires to various ports of a channelized subsheet
An approach that works:

Inside digital_interface.SchDoc, group the pins as shown in Figure 1.


Figure 1: Digital pins grouped into two busses. Each bus will be connected to one input of the channelized block.

Then, in the top level sheet, just use standard channelization as shown in Figure 2.


Figure 2: The CSAb and CSBb busses are channelized in the usual way.

This isn't as elegant as I'd like, but it works.