I think this would be very easy to made (like in C# just Dictionary<string,List<string>>) and very useful. Currently there is virtually no way how to make it. For example, in my game I have NPC selling variable number of items. With this extension, I could simply iterate through the all strings under key (NPC ID). Or say you want to store variable number of information (also it can be parsed and contain anything) for many objects - again, you could add and read items from lists under unique identifier. The key thing is not fixed lenght (thus the list and not array).
Magic deque cannot be used as the array position is too limited value-wise.
The key could be int or string, probably string would be more useful but a bit slower (do not think we would notice difference unless there are thousands of elements)
Add element under key
Delete element under key
Delete element under key by index
Insert element at position //could be useful
Clear list for key
Does list under key contain an element?
Is the list for given key empty?
Get count of elements under key
Get element under key by index
Get number of keys
Find element under key //Returns index
I could pay something for this, its very easy one. I cannot write it myself as I know just C# and have no time to study MMF2 SDK.