you cannot enter the IFR system from any point there are pre defined entry points
You've got me a bit confused.
Departing a "farm strip" (Class G), for an international or other
Eurocontrol IFR flight, one files the flight plan, gets airborne, calls up the regional FIS unit (London Information usually) and they give you a provisional airways clearance (which, them being non-radar etc, will be a squawk and the IFR controlling authority frequency - usually London Control). Then you change to London Control and they will clear you to enter CAS in the climb on a track to some waypoint which they will give you. This could be anywhere where it suits them.