The proper way to get DC-coupled bipolar output is to have an external bipolar power supply and an active circuit to shift and scale the voltage accordingly (see posts on this forum for "eurorack" and they should have some example circuits).
You could also try to set one of the analog outputs to 0.5 and thus get 2.5V at its output to be used as your reference instead of ground. However, the analog outputs have a 220ohm resistor in series (to protect them from overcurrents), so the effectiveness of this solution is going to be dependent on the input impedance of the scope (which should be pretty high anyhow, so it could work). Also, the two circuits should not have a common ground (e.g.: Bela should not be earthed, either directly or, e.g.: through your laptop).
When using 4 or less channels you can get a 44.1kHz sampling rate.
All in all, though, I am not sure why you need bipolar outputs. In his description of the workshops (e.g.: here), Derek mentions Bela, so perhaps he is the best person to ask in this respect.