This is actually a bit of a tricky thing: A scout's flash can only be written from the bootloader (consider what would happen if you would write to flash from within the application itself while also running the same application - at some point you'd overwrite the currently running code and things would end up badly). However, the bootloader doesn't know how to talk to the wifi backpack, so it cannot receive data directly through wifi.
What should be possible is to let the lead scout do an OTA update of a field scout (through mesh OTA) and then let that field scout clone itself back to the lead scout. This should be possible with the current ota commands, though you'd have to call the (IIRC)
ota.end command from a timed delay, since you cannot currently find out when
ota.clone is completed. For more info on these ota commands, see the changelog - this is still fairly new code, and subject to change, so it's not contained in the reference documentation yet.