There used to be sets of lock plans by "Pryor Mountain Bill Newton". Had various locks including back and foreward action mule ears.
Did a quick google, and found h ttp://pioneerarms.com/mule_ear_lock.html.
A mule ear is definitely something that can be shop made. About the only issue is that either the lockplate has to be quite thick, or there have to be lugs on the inside, to allow for hammer and sear pivots.
