I think that Slashdot ate some of your formatting (angle brackets?). I'm not sure how the hash table is used for messaging (you might want to consider Unix domain sockets for messaging, depending on what's going on). But...
You can basically grow shared memory segments on the fly. They're shared in the FS page cache, so if you create a "new" memory mapping on the same backing file that's just bigger, it'll contain all the same stuff in the original segment as the original mapping.
So you just:
a = mmap(..., 2 megabytes,...)
(Use for a while, realize that it's too small)
old = a
new = mmap(..., 4 megabytes, ...)
a = new
You have to do that per-process, but you can either have the initial part of the memory indicate the mapping length (every time you use it, check the length and reallocate if you're off) or you can use a semaphore or socket message or other OOB message to indicate when to resize.
I don't see how this is (theoretically) any harder in C++ than in Java as far as the problem specifics go (I mean ignoring general reasons that C++ is harder than Java).
Also when they say that a python Manager object is slow, access to it should be considered similar to access to a synchronized method in Java. But reading large amounts of data over it can be problematic, depending on exactly how you're using it.
Depending on your access needs and where your bottlenecks are, I'd also consider using memcached or something like that if you think you might ever expand to multiple machines.