This works as desired for me: 
void modSV( SV *ref ) {
    SV *sv = SvRV( ref );
    unsigned __int64 uv = 9223372036854775809ull;

    sv_setuv(sv, uv);
}

[download]
Outputs: 
SV = IV(0x381af90) at 0x381afa0
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK)
  IV = 123

SV = IV(0x381af90) at 0x381afa0
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK,IsUV)
  UV = 9223372036854775809

[download]
Haven't yet investigated why/where your approach falls down.

Cheers,
Rob

In reply to Re: XS: returning a 64-bit unsigned int? by syphilis
in thread XS: returning a 64-bit unsigned int? by BrowserUk

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