I was wondering if it practicable to have an C++ standard library compliant allocator
that uses a (fixed sized) buffer that lives on the stack.
Somehow,
It really depends on your requirements, sure if you like you can create an allocator that operates only on the stack but it would be very limited since the same stack object is not accessible from everywhere in the program as a heap object would be.
I think this article explains allocators it very well
http://www.codeguru.com/cpp/cpp/cpp_mfc/stl/article.php/c4079