How to do a post frontmatter prop mapping step?

[L0laapk3]

My goal is to perform some additional server-side parsing on the props. So far I have this working 'solution', which required hacking in an export for default_frontmatter:

svelte.config.js

const config = {
	preprocess: [
		mdsvex({
			frontmatter: {
				type: 'yaml',
				marker: '-',
				parse: function(value, messages) {
					let result = default_frontmatter(value, messages);
					result = propMap(result); // the magic
					return propMap(result);
				}
			},
		}),
	]
}

How do I achieve something like this without having to patch mdsvex?