This has been driving me nuts, but I now have it all working. It's probably not very streamlined - for example it adds a new spi device for driving the LEDs where in reality that device *may* be similar to that in one of the rb400's and so might be re-usable... but to be honest I've spent so much time on this that I need to get on with other things. Similarly, some of the naming is a bit off ( so that spi is called rb400-spi-misc for example, because it's also used in the RB4xx series and that was how it was named in Mikrotik's patches. )
As far as I can tell, Mikrotik's bootloader is buggy, so you have to replace it ( link and instructions in my patch dir. ) This also means that you need a TP-Link style image for the 750r2, which to be honest is no bad thing. However, it also means that the bootloader doesn't set some things up correctly ( took me more than a day to work out why I couldn't get the LEDs to work. ) I don't have a QCA9533 datasheet, so my patch currently just writes a load of registers to the values they have on boot with Mikrotik's bootloader - most of these are probably not needed but again I'd lost the will a bit by that point!
One interesting thing is that the Breed bootloader can write the entire flash in less than a minute, whereas OpenWRT takes more than 5 minutes using dd or mtd write - would be good to know why.
Anyway, I'm hoping that a "real" OpenWRT developer can take this and use it to create "proper" support in OpenWRT - but for now this works and is relatively easy to do... it's just a bit messy.
Instructions, code and patches here. Usual caveats apply about being careful not to brick your router and me not being liable for it . Probably best if someone experienced tries it first and feeds back....