You right, we have no example of a pure DefaultMemoryAllocator or user applied memory control policy.
But the MPEG2 demuxer may be used as an example of such an approach.
Please look at umc_demuxer.h and umc_demuxer.cpp.
The demuxer class member m_pMemoryAllocator is a MemoryAllocator (not directly used because this is an abstract base class) used to provide user defined memory for the frame constructor.
This member will be created as a DefaultMemoryAllocator object and then used by the frame constructor.