I think there are several problems to what your wanting to do.
1. With the heater control valve off, there will be no flow.
2. If there is flow (control valve on), it will come from the cold side of the heater core.
3. The extension on the OEM fitting, is there to help heater core flow, by acting as a siphon, helping to lower the pressure on the return side.
I think a better solution, would be to find/make a fitting for the water neck, but it won't give accurate reading until the thermostat opens. To get around that, I think they make thermostats that have bypass holes in them or you could drill one.