Hi Neil,
Most of the PC_PRINTER library routines are supported in .NET as well as native code although some of them, (i.e. PC_PRINTER_INFO) behave a bit differently because a different coordinate system is used in .NET print functions.
The PC_PRINTER_DEFAULT_PROPERTIES that you are using should work in .NET as well. If you are finding that this does not behave correctly then I would suggest that you open up a support incident with Customer Care so we can look into this.
Thanks.