The usbhost support isn't in a PR yet, needs a lot of cleaning up first. Currently it's just exposing the host drivers built into tinyusb; I don't know if they have host support for midi built in. Once this initial support is finished and working it should be possible to extend it to provide a runtime / python definition interface for custom drivers like USBDevice has but I'm not sure how much work that'll be.

@andrewleech what do I need to add in my mpconfigboard.h if I want to test your tinyusb implementation on my stm32h7 (REPL over USB and MSC support for example?)
I added:

#define MICROPY_HW_TINYUSB_STACK 1
#define MICROPY_HW_ENABLE_USBDEV 1
#define MICROPY_HW_USB_CDC 1

but I cannot see the USB device in my devices list after a successful build and deployment.

Is this with the current version of my PR checked out?
I would have expected just #define MICROPY_HW_TINYUSB_STACK 1 should be all it takes.
That should get USB CDC for repl; mass storage isn't supported currently.

Maybe there's an issue with the h7, I'll have to see if I can find a high speed USB device to test with.

No need to test anymore, I forgot to add first #define MICROPY_HW_ENABLE_USB 1. I thought the #define MICROPY_HW_TINYUSB_STACK 1 did this by default.
So I have REPL over USB working now with:

#define MICROPY_HW_ENABLE_USB 1
#define MICROPY_HW_TINYUSB_STACK 1

(yes, tested with your version of the PR checked out.)
Is it straight forward to add MSC here?

I don't know how much effort MSC would be, I hate it on embedded devices because it pretty much always results in corrupted flash.
I know it can be enabled in TinyUSB but then it'd need binding code to get from that to the raw flash block layer.
I and a couple of others have been slowly working on a USB MTP driver which is the much safer replacement for MSC, hopefully that can be finished off soon.