There is a lot of work around external memory allocation in Node.JS at the moment as you can see here: joyent/node#4964
It would be great to track the progress and see how things are evolving and if it needs changes in the BinaryJS code.
Some leaks are observable currently with Node 0.10.15 when exchanging a lot of data with multiple clients.
Things are already way better in Node 0.11.4 (memory consumed divided / 2 for the same kind of exchanges).
Whatever issue you're experiencing was probably fixed in joyent/node#5720