Sorry, I didn't see the question. Yes, in theory you should be able to flow a heater core from the inlet and outlet of the radiator. Flow will vary based on things like hose size, port resistance, parasitic drag, pressure differential, etc... But it should work. If you can find a Y fitting and point it the right direction (with flow) it should work better.
Like this...
Thanks. I think I'll give it a try and see how it goes. What's the worst that can happen... no heat? Already there!