If it were me I would get a junkyard rear A/C / heat unit out of a van / surburban and a 12 volt circulator pump out of most things German (usually after run coolant pump) and plumb that in to the hottest spot you can find on your coolant loop.
I bet that if you plumbed the return back to the cold side you could get heat for quite a while after engine shut down.
A better but more expensive solution would be to add an insulated heat storage tank and pull from that all night.
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