Well... I don't know whether to be happy or even more frustrated...
After trying some other ISOs (all unmodified!), I somehow got it to work, then switched back to the ISO(s) I've tried before - and could never reproduce to problem!! Strange. Very strange!
Something tells me you're gonna hear from me again :)

Anyways, as for what I was doing before (trying for hours to avoid the need of steps 7-12):
1. Extract boot.wim and mount it
2. [Elev. CMD] Dism.exe /image:C:\Mount /Add-Driver /driver:C:\Drivers /recurse
[C:\Drivers contains x86 versions of:
iusb3xhc.inf iusb3xhc.cat iusb3xhc.sys iusb3xhc.man
iusb3hub.inf iusb3hub.cat iusb3hub.sys iusb3hub.man ]
3. Unmount, add boot.wim back to ISO.
4. Copy ISO to _ISO\WINDOWS\WIN7; Make the ISO contiguous
5. Boot from USB and start Windows 7 setup.
6. Click on Install Now. Setup starts (no "blue command window" appears) and the missing driver error appears after few seconds.
7. Plug in another USB with the same drivers on it.
8. Click Browse and select the folder containing the drivers.
9. Select both drivers and click Next
10. Error message appears...
11. Click X to go back to the first setup screen.
12. Click Install Now... Setup run flawlessly.

Thanks again!