Putting a plug in the T leaves it open to future use. Go for it.
I find 50 ft is more like the magic #. (A coil in a garbage can would work but how you going to drain it?
heatercore in a bucket)
And how good of heat transfer your pipe has matters to. Is it up along the ceiling, outside, underground? Copper?
My point is that having a good cleanout/drop before heading into the wall/manifold/filters lets you purge the lines to keep it a dry system when it really matters. I purge my lines while I'm letting things flash if doing more than a panel or two at a time.
And I wish I'd of plumbed a shut off after that drop but before the 'manifold'
