You could arguably keep your MD driver's license and carry the PA Vehicle Code along with you when you finally do get stopped. In there it states that if you meet the requirements for both registration and insurance, you are allowed to operate a vehivle in PA.
You will be able to get a PA title and register your mopeds in PA if you get a PA Drivers License first. Then you will need to find the correct forms on PENNDOT's website to transfer title/registrations of your vehicles into PA:
Of course you will need to plead your case about not needing titling or registration of any kind in MD, so a copy of the MD Vehicle Code will be needed (even though PENNDOT employees have it at their disposal).