Might we persuade the JAA to pass a rule saying:
"A JAA pilot can use the privileges of a FAA IR on a JAA registered aircraft on a non commercial flight."
The FAA IR holders will be doing what they do anyway but no longer under 'N' reg, so less hassle for all concerned.
I'm sorry but an FAA IR does not equip you well enough to fly in European airspace.
Mind you neither does a Spanish one to fly in the South of the UK in IMHO.