Hi Rajeev,
jQuery library comes by default in MII from 14.0 SP05 onwards as part of the SAP UI5 library which is loaded by bootstrap.js. Hence the jQuery function getJSON should work in MII 15 SP03 as well.
Is there some error in the console on using the function, if so please share the same.
If you could be more specific on the scenario with MDO in UI may be that will help answering your query. Other MII experts please share your views here.
Thanks and Best Regards,
Ria