FWA NATCA:
Remember ALTRVs are only in FAAO .65, an ALTRV is not recognized by ICAO and most member states. While there is a PAC Military Altitude Reservation Facility and a EuCARF that coordinates ALTRV requests (mostly US Mil), only in the US can ATC give an ALTRV clearance that remains in effect take-off to IFPFP or landing. Elsewhere, an ALTRV is really just a request that is handled by each ACC. And there are two types--stationary and moving.
Are Block Altitudes available outside the US? I have gotten them in CA, but never tried in Europe.
GF