I *think* it should be possible, as I *think* the only difference between the LE and the 'normal' 220 (not the plus) is the flashrom. So overwriting the flashrom with a copy from a 220 could upgrade (or downgrade, depending on your point of view) it to a 220.
Unfortunately it's not that simple to overwrite the flashrom. AFAIK the only way to write the flash in one write action is using a JTAG cable, or maybe a serial cable, when the bootloader supports a wholeflash. From within the Linux system the flashrom is divided in partitions, and 'simply' overwriting is only possible when the partition sizes are equal between both systems, and have the same purpose.
You might just throw a dice and pull the version check from a fw 3.12 update usb stick, to let it flash your box, but be prepared for a bricked box.
There appear to be the firmware a query whether a certain file matches
Actually not. The check is:
Is_NSA220=`mrd_fbits | grep "D1 01"`
AFAIK this reads some bytes directly from flashrom, not from a file.
Which functionality do you want to add to your LE? Can't you just add it using a FFP-stick?