peek/poke or other method to write direct into nrf52 registers?

I just tried to unlock flash and write to the NFCPINS register with the latest build 2017-11-27 :

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>var f=require('Flash')
=function () { [native code] }
>var NFCPINSreg=0x1000120C
=268440076
>var NFCPINSdata=[0xff,0xff,0xff,0xfe]
=[ 255, 255, 255, 254 ]
>E.getFlags()
={ "deepSleep": 0, "pretokenise": 0, "unsafeFlash": 0 }
>E.setFlags({unsafeFlash:1})
=undefined
>E.getFlags()
={ "deepSleep": 0, "pretokenise": 0, "unsafeFlash": 1 }
>f.read(4,NFCPINSreg)
=new Uint8Array([255, 255, 255, 255])
>f.write(NFCPINSdata,NFCPINSreg)
=undefined
>f.read(4,NFCPINSreg)
=new Uint8Array([255, 255, 255, 255])

Despite the flag unsafeFlash being set to 1, it seems that the flash register did not change. Any idea?