Legally, the title of Holy Roman Emperor can only be conferred by the Poper - following the "Donation of Constantine."
The modern stumbling block would be that the "Donation" is widely acknowledged to be a fake, but His Majesty King Juan Carlos could fall back on the fact that the Church continued to acknowledge the Emperors, despite the fact that the Canon lawyers would have been aware of this at least from the Renaissance.
http://en.wikipedia.org/wiki/Donation_of_Constantine
Given that the forgery is apparently from the 9th Century that means that Charles the Great much have been annointed of the Pope's own authority.
So, anyway, all hail his de facto Imperial Majesty Emperor Juan Carlos, I suppose.
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