I think it's logical move; there's already several modes of 'abbreviated' callsigns and this would make it, as has been said, more obvious to the ATCO, FISO or Radio Operator.
I'm no longer a member of the UK Phraseology Working Group (which decides what goes into CAP413) and as far as I'm aware, AOPA don't sit on the group (maybe they should), so it needs someone who does sit on the PWG to introduce it for discussion so the best I can suggest without further research is e-mail
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