base64url encoding in message template function

[irina-gherman-d]

any ideas if I can define my own message template function? 

i am having the following code, defined for a HMAC signature calculator. 

 <AssignVariable>
        <Name>hmac.stringToSign</Name>
        <Template>{request.header.X-Request-DB-ID}:{encodeBase64(sha256Hex(request.content))}</Template>
    </AssignVariable>

 I would like to use a base64Url encoding instead of base64 without having to do too many changes in my code.  I simplified the use case here to get to the root of the problem. 

The template is defined in a PropertySet and is read from there. The string to sign is a concatenation of many other fields/headers and data.