I am attempting the same thing currently. At first I used the same approach for converting to hex: 

ClassMethod Hex(pRaw As %String) As %String [ Internal ]
{
set out="", l=$l(pRaw)
for = 1:1:{
set out=out_$zhex($ascii(pRaw,i))
}
quit $$$LOWER(out)
}

But have discovered that the Hex function above is not correct. Instead use the system function,  ##class(%xsd.hexBinary).LogicalToXSD.

##class(%xsd.hexBinary).LogicalToXSD

The conversion to hex using the custom method was removing zeroes in the string. That had a cascading effect, the canonical request hash will be wrong, and consequently so will the string to sign and then ultimately the signature. Hope this helps anyone else that might be struggling. 

Here is some documentation specifically for S3:

https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-header-based-auth.html

I tried running your example locally, and do not get a result of 1, as you noted in your results. No value is actually returned. The approach you listed does not work.

> data.invoke_method(o, '%GetData', 1)

{ ok: 1, oref: 4, method: '%GetData', result: '' }