Hello Charlie,
Did you get this working? I am trying same thing: boot on a SD and then LE on the usb. 
I followed all the steps I found on the internet and now I only have to modify the cmdline.txt.
My problem is that I do not kown which label I must include. I tried to connecto via ssh to the LE (before I perform the changes) to see which label is assigned but I am not able to connect with ssh.
This is the original content:
      boot=/dev/mmcblk0p1 disk=/dev/mmcblk0p2 quiet
Did you try with this?
      boot=/dev/sda1 disk=/dev/mmcblk0p2 quiet
Any success on your side?
Regards,
pblgmz
[hr]
I do confirm IT WORKS!!
The line I wrote on my previous post WORKS.
Regards,
 
		 
		
		
	 but I do not get it working
  but I do not get it working  . To be honest I get the green mark for the CA at tvheadend and sometimes images appear for 1 second... that's all
 . To be honest I get the green mark for the CA at tvheadend and sometimes images appear for 1 second... that's all  .
 .