you are trying to do the impossible. With a manifold system you can ONLY circulate through the manifold. The individual lines should be short enough so the hot water in the manifold arrives quickly, but NOT "instantaneously". "Radiant heat header/manifold" would be useless and completely ineffective for your purposes, because every one of those "circuits" loop back to the pump at the boiler. If you tried to use them for a hot water system you would create an IMMENSE can of worms for yourself, along with a lot of expense.