emonTX plus USB plus batteries (as backup)

Having searched the site for this I had found the emonTX wiki http://wiki.openenergymonitor.org/index.php?title=EmonTx_V3 which quite clearly states "DO NOT MIX POWER SOURCES".
I understand that if I'm going to use the AC-AC for the power reference and a USB PSU I must remove J2 and the wiki also states "please make sure batteries are removed before connecting USB".
I get that it means you cannot have USB attached and still have batteries installed too, so this is more of a development suggestion.

How difficult would it be to allow the batteries to be installed AND USB PSU attached, with J2 off of course, and basically disconnect the battery supply from the main circuit as long as USB power is present but then use the battery supply when USB is absent, like during a power failure. Of course if there's a power failure you won't be monitoring power usage with the CTs but if you have temperature sensors attached, and you base is still powered (UPS or something) then at least you will still be monitoring temp.
The catch with my reference to disconnecting the battery supply while USB is present would probably mean some additional changes to still allow for the battery monitoring while the USB is powering the unit, else you stand the risk of not knowing when the batteries need changing until it's too late...