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; } }