How to use WEBC to generate XML?

[ifireball]

I'm trying to make *.xml files in my repo be handled by WebC. This is what I tried so far in .eleventy.js:

const { EleventyRenderPlugin } = require("@11ty/eleventy");
// ...

module.exports = function(eleventyConfig) {
  //...
  eleventyConfig.addPlugin(EleventyRenderPlugin);

  eleventyConfig.addTemplateFormats("xml");

  eleventyConfig.addExtension("xml", {
    outputFileExtension: "xml",

    compile: async (inputContent) => {
      let output = await this.renderTemplate(inputContent, "webc")

      return async () => {
        return output;
      };
    }
  });
}

This fails with: TypeError: this.renderTemplate is not a function

I also tried to just set key to webc and outputFileExtension to xml and got:

An attempt was made to override the *already* overridden "webc" template syntax via the `addExtension` configuration API. A maximum of one override is currently supported. If you’re trying to add an alias to an existing syntax, make sure only the `key` property is present in the addExtension options object.

How do I make this work?