B747-400 FLC ASKS TOKYO RADIO FOR A 20 NM DEV AROUND TSTMS AHEAD OF ACFT. TOKYO CTR; RJTG; DENIES REQUEST; '...UNABLE.' AFTER SEVERAL REQUESTS; CAPT ELECTS TO USE HIS COMMAND AUTH AND DECLARES AN EMER WITH AIRLINE APPROVED PROCS USED IN HIS DIVERSION. RPTR COMPLAINS OF NO ALTERNATIVES BEING OFFERED BY ATC.

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ENRTE TO SFO; JUST PRIOR TO OAK FIR; THE RADAR WAS PAINTING WX AT 12 O'CLOCK POS (APPROX 80 MI). WE ASKED FOR A DEV 'R OF COURSE UP TO 20 MI FOR WX;' FROM TOKYO RADIO. AFTER SOME DELAY; TOKYO RESPONDED WITH 'UNABLE DEV.' WE INFORMED TOKYO THAT WE MUST DEVIATE AROUND WX. THEY RESPONDED BY SAYING UNABLE DEV AND TO STAND BY. AFTER ANOTHER DELAY; TOKYO RADIO CALLED US BACK AND STATED 'UNABLE DEV.' WE INSISTED THAT WE MUST HAVE A DEV FOR WX. THEY AGAIN INSISTED ON NO WX DEV. AT NO TIME DID THEY SUGGEST AN ALTERNATIVE PLAN OF ACTION; IE; CLB OR DSCNT WITH DEV. SO; IN ACCORDANCE WITH ACR FLT OPS MANUAL WE ACCOMPLISHED THE FOLLOWING (DEV REQUIRED WITHOUT CLRNC): 1) TURNED ON ALL EXTERIOR LIGHTS. 2) IMMEDIATELY INFORMED TOKYO RADIO OF OUR INTENTIONS TO DEVIATE. 3) BROADCAST OUR POS AND INTENTIONS ON 121.5 MHZ. 4) STARTED DEV LATERALLY TO THE R. 5) A DEV OF AT LEAST 10 MI WAS PLANNED SO WE CLBED 500 FT (EBOUND; R DEV). 6) NO CONFLICTING TFC WAS OBSERVED VISUALLY OR ON TCASII. 7) AT APPROX 10 MI; IT WAS DETERMINED THAT WE COULD RETURN TO COURSE. WE RETURNED TO COURSE AND DSNDED BACK TO ASSIGNED ALT. WE ADVISED TOKYO THAT WE WERE BACK ON COURSE. I SUSPECT TOKYO WAS COORDINATING WITH OAKLAND; CAUSING DELAY IN REPLY. CONCLUSION: DUE TO THE LARGE VOLUME OF TFC OVER THIS RTE AND THE REQUIREMENTS TO AVOID DANGEROUS WX FOR SAFETY REASONS; I THINK SHORTER RESPONSE TIMES TO WX DEV REQUESTS ARE NECESSARY. ALSO; ATC SHOULD; IF POSSIBLE; SUGGEST A COURSE OF ACTION.