Limit Regarding Lookup in MS Dynamics CRM 2013

So Today we had an interesting requirement: We had a custom entity, called Paper, for which we wanted to limit the entities that could be selected for the regarding lookup.

I can't come up with a supported way of doing this so this is the unsupported way:

In essence, this will limit the Regarding to new_keyissue and new_goal.

You'll need to fire it on the onChange event like this, NEW.Paper.limitRegardingLookup.

if (typeof (NEW) == "undefined")
{ NEW = {}; }
NEW.Paper = {
    limitRegardingLookup: function () {
        
        var KeyIssueOTC = GetEntityTypeCode("new_keyissue");
        var goalOTC = GetEntityTypeCode("new_goal");
  
        var ObjectTypeCodeList = KeyIssueOTC + ", " + goalOTC;
        var LookupTypeNames = "new_keyissue:"+ KeyIssueOTC + ":Key Issues,new_goal:" + goalOTC + ":Goal";
        
 Xrm.Page.getControl("regardingobjectid").setFocus(true);
  
        document.getElementById("regardingobjectid_i").setAttribute("lookuptypes", ObjectTypeCodeList);
        document.getElementById("regardingobjectid_i").setAttribute("lookuptypenames", LookupTypeNames);
        document.getElementById("regardingobjectid_i").setAttribute("defaulttype", KeyIssueOTC);

    }
};

function GetEntityTypeCode(entityName) {
    try {
        var lookupService = new RemoteCommand("LookupService", "RetrieveTypeCode");
        lookupService.SetParameter("entityName", entityName);
        var result = lookupService.Execute();
        if (result.Success && typeof result.ReturnValue == "number") {
            return result.ReturnValue;
        }
        else {
            return null;
        }
    }
    catch (ex) {
        throw ex;
    }
}