The conversion from byte array to the binary data required should be simple:
kDate = Utilities.computeHmacSignature(Utilities.MacAlgorithm.HMAC_SHA_256,
'20130618', 'AWS4' + kSecret);
kDate = Utilities.newBlob(kDate).getDataAsString();
kRegion = Utilities.computeHmacSignature(Utilities.MacAlgorithm.HMAC_SHA_256,
'eu-west-1', kDate);
BUT you have to look onto this open issue in the bugtracker - there could be some issues in conversion.
maybe you could try to make a String.fromCharCode() loop and avoid negative numers:
kDateB = Utilities.computeHmacSignature(Utilities.MacAlgorithm.HMAC_SHA_256,
'20130618', 'AWS4' + kSecret);
kDate = '';
for (var i=0; i<kDateB.length; i++)
kDate += String.fromCharCode(kDateB[i]<0?256+kDateB[i]:0+kDateB[i]);
kRegion = Utilities.computeHmacSignature(Utilities.MacAlgorithm.HMAC_SHA_256,
'eu-west-1', kDate);