I'm wondering, if I simply connect
my laptop to it via an HDMI cable and set Pd to use HDMI for audio output,
will I automatically have the ability to send specific channels of audio to
any one of the 7 speakers?
This should work.
Or would it require additional hardware or
software? If so, what?
Probably not. It depends more on the hardware and any special drivers that might be required to use it with your OS. Pd only talk to the sound devices that are given to it by the OS. We have used Pd with RME MADI interfaces at work which address 64 or 128 channels.
Note, that you get the "raw" channels directly. Surround or ambisonic mixing is up to you.