The part about inplace transofrmations is irrelevant to the question and redundant. You are asking how to deal with peer filters/pin that don't implement IMemInputPin::GetAllocatorRequirements
. From MSDN:
The input pin is not required to implement this method. If the filter has specific alignment or prefix requirements, it should implement this method.
Implementation of this method is not mandatory. This means that on your output pin you are free to configure the memory allocator at your own discretion, no need to consider peer pin opinion on the allocator properties.