BACnet defines a multitude of transport protocols besides TCP/IP. So you will most likely (since it is a mature protocol) find the bulk of devices communicating via something else than TCP/IP.
The specification mentions a BACnet Broadcast Management Device, which would be most likely the communications endpoint talking to Bluemix.
Using the IoT Foundation Service in Bluemix, you can reach Bluemix trough: TCP, UDP, MQ, MQTT, HTTP - basically anything that sends IP.
Have a look at the Bluemix service NodeRED that gives you head start.
What I would do:
- NodeRed runs on PC or Raspberry PI too - use Thingbox for the PI
- The PI has been shown to run BACNet
- You even can buy BacNET for PI
- Now the tricky part (we could collaborate on that): build some Nodes for NodeRED to do the BacNET interaction
With this approach you decouple the local BacNET network from the cloud. I would then use MQ(TT) to talk to the cloud, so you don't loose data when there is a connection challenge (e.g. you use 3G).