Is there a way to post a raw RFC 5322 message to the create message endpoint?

I have an RFC 5322-formatted message, and I'm trying to POST it to the /users/{id | userPrincipalName}/mailFolders/{id}/messages endpoint.

Per the docs for that API, that is possible when POSTing a request to that endpoint with a content type of text/plain. Specifically, this note in the "Request headers" section for Content-Type (emphasis mine):

Use application/json for a JSON object and text/plain for MIME content.

Combined with this additional note in the "Request body" section:

When specifying the body in MIME format, provide the MIME content with the applicable Internet message headers ("To", "CC", "BCC", "Subject"), all encoded in base64 format in the request body.

I can't find a way to do this with the SDK. Am I missing something, or does the SDK not support this?

I'm able to get as far as this:

client.
	Users().
	ByUserId(userID).
	MailFolders().
	ByMailFolderId("inbox").
	Messages().
	Post(
		ctx,
		// This doesn't compile as it isn't `model.Messageable`.
		// How do I send this?
		base64.StdEncoding.EncodeToString(email),
		nil,
	)

A model.Messaage gets sent as application/json, and can't be built from an RFC 5322 message (as far as I can tell).

Start with the Messages().Post entry point in the SDK and compare its model.Messageable parameter with the documented /users/{id | userPrincipalName}/mailFolders/{id}/messages endpoint. Check whether the request path can represent base64-encoded RFC 5322 content with text/plain; done means establishing SDK support or documenting the missing request path.