Obtaining 12 V from a 20-250 V DC source
The schematic is generally speaking, working. But there are some points that need some clearing:


The rectifier circuit is important and have to be designed to work in whole frequency range of the generator.
The power dissipated from the upper transistor (in worst case) is: 190V*50mA=9.5W. For lower transistor: 2.4W; It is not something not seen, but it will need not very small heat sink in all cases. In my opinion you should think about making the consumption lower. If you can lower the current consumed to 5mA, the transistors will not need heat sinks at all.
Another possible approach is to use small accumulator battery for the start-up process and to charge it when everything works. This way the efficiency of the system can be higher, and considering the size of the heat sinks, the size smaller.