Cutting out a piece of SS railing

Depending on size of your rail as well as its structure in the location of the new installation you're considering, quite likely there is hardware out there that can let this be an easy job performed at your slip.

Off the shelf pieces with the line made to size with crimped fittings. Call a sailboat rigger. All that stuff is standard on a sailboat lifeline. Installation will take all of 15 minutes. Or if you're handy play around on these pages and do it yourself. https://www.sailrite.com/All-Hardware/Boat-Hardware/Lifeline-Hardware http://www.ddthreadsplus.com/outside-eye-end-heavy-duty-2-set-screws

New stainless crimpless hardware out there, too. Just replaced by lifelines with no crimping....all stainless...

I m not sure what you meant about replacing the section by an SS Cord.

Any decent welder should be able to cut the section, weld a hinge on one end and a latch on the other. Depending on the location you want it to be solid when closed

Some marinas do not allow welding for safety reasons so you may have to move the boat to a yard for a day but you don’t have to haul out.

ditto on the hinge solution

Mid 2000 Carver 530's had a section which would slide forward inside the rail so you could access the starboard side door, then it would slide back in place. It also had a twist lock built in so it would lock in place like a extendable boat hook. Pretty simple and sleek design.

https://www.suncorstainless.com/quick-attachtm

This was a resource I used...you could make a bridge with a pelican hook and cable with a toggle to make a cable bridge in the new rail opening...depends on its overall structure surrounding the new opening. My issue with a hinge is that round rail folding to round rail is a bit sloppy and tends to face distortion and failure.