Setting dynamic voice attribute inside TwiML Bin from Twilio Studio via TwiML Redirect

I'm trying to use a variable from a Studio Flow to set the type of voice to be used by a gather input interaction generated by a TwiML Bin, but when I attempt to reference the variable as the definition of the voice attribute I get a syntax error. I know that the {{VoiceEnglish}} variable reference works correctly since it will say the correct value if I put it inside of the Say tags, but is there no way to use it for any attributes? Example of what I would like to work is below:

<?xml version="1.0" encoding="UTF-8"?>

<Response>
  <Gather input="dtmf" timeout="15" numDigits="1" action="https://webhooks.twilio.com/v1/Accounts/[AccountSID]/Flows/[FlowSId}?FlowEvent=return" method="GET">
    <Say voice="{{VoiceEnglish}}" language="en-US">English verbiage here</Say>
    <Say voice="{{VoiceSpanish}}" language="es-MX">Spanish verbiage here</Say>
  </Gather>
</Response>

I think that the TwiML syntax is reported as invalid because it cannot tell that the result of {{VoiceEnglish}} or {{VoiceSpanish}} will be a valid voice.

You are able to to save the TwiML Bin as long as you agree to the modal:

Once you save it, you can use it in your Studio Flow as long as you provide valid VoiceEnglish and VoiceSpanish parameters in the redirect URL.