Check to see if that 4 foot wide "U" goes across a doorway or hallway on
the floor plan directly above that "U".
You can use 3/4 inch sharkbite fittings for this, but I've never heard
of a flexible supply hose being allowed to stand in for a pipe in a hot
water heating system.
Now, if this is a house with only one thermostat, then you almost
certainly have gate valves on all but the longest radiator loop. These
gate valves are called "balancing valves", and their function is to
pinch off the flow through the shortest straightest radiator loops to
force more water through the longer and more tortuous radiator loops,
thereby promoting uniform heating of the house.
If you put another coupla bends in that radiator loop, it's going to
offer more resistance to water flow through it, and so you'll have to
rebalance the flow of heating water through all of the radiator loops.
This would involve closing the balancing valves just a bit on the
shorter straighter loops to force more water through the loop that you
added the elbows to.