OK... I've done some research elsewhere on this and have found the following info, in case anyone else is interested.
It is possible to use BVG money to finance self employment, once you have been accepted as self-employed by the AHV.
However, if you are contributing to the AHV pot via any other means, eg if you are employed part-time and are self-employed for the remainder of your time, it is not possible to get access to the fund.
Sadly, the latter applies to me! I would personally rather use some of my meagre pension fund to help my self-employment now with a view to replacing it over the coming years once my business is a runaway success

... but there ya go...