Implementing custom grant types in DotNetOpenAuth

Question

Using DotNetOpenAuth, we have successfully implemented several standard OAuth 2.0 grant types on our authorization server.

We now would like to implement a custom grant_type, preferably without modifying the DotNetOpenAuth source itself.

I could handle it manually like this:

public async Task<ActionResult> Token()
{
    if (Request["grant_type"] == "my_custom_grant")
    {
        if(InvalidClientCredentials)
            return ErrorJson();

        var user = FindUserWithSpecialCode(Request["special_code"]);

        if (client.IsAlreadyAuthorizedForUser(user))
            return Json(new { access_token = "", token_type = "bearer", expires_in = 1800 });
        else
            return ErrorJson();
    }

    HttpResponseMessage response = await AuthorizationServer.HandleTokenRequestAsync(Request, Response.ClientDisconnectedToken);
    return response.AsActionResultMvc5();
}

But it's a bit hacky and I'm worried about security.

I've seen custom grant types are supported in other OAuth libraries, but is this possible with DotNetOpenAuth?