The port does appear to be too shallow, though it may have been designed this way to prevent damage to the camera if the micro-HDMI cable is bent.
For future versions of the camera, something similar to Atomos' proprietary locking mini-BNC connector on the original Samurai would be preferable. There may be licensing issues with this however, and a mini-SDI port would protrude from the camera, which may make the connector more likely to get damaged when thrown in a bag. It also kind of spoils the clean lines of the design.
On initial tests, an Atomos Samurai connected to the BMPCC via an Atomos HDMI-SDI battery converter dropped out every few seconds. Subsequent tests with a new cable (
http://www.amazon.co.uk/gp/product/B0051YHVPM) were much better. I wiggled the micro-HDMI adapter cable quite a bit and didn't get a drop out, so I feel happier using it.
I now think using an external recorder for long-form event work is a viable option, but I would always want to be recording to SD card at the same time as a back-up.