Welcome aboard and congrats on your Hunter 20. Nice little boat as we really enjoy ours. I really don't think you can replace the keel line without dropping the keel on the 20 because the attach point on the keel remains up in the keel trunk even with it dropped down vertically. Even if you could fish a line up through from the bottom I would not recommend just tiying a knot in the end of a line as I can't imagine that lasting very long on a 400lb swing keel. You really want a thimble in the end of line attached to a pinned shackle in the keel.
I was able to remove our boat from the trailer by myself in about 30 mins. I then modified my motorcycle jack to be a keel jack and lowered the keel down out of the boat. I discovered that two of the four keel bolts were bent from a side load and replaced them. I repaired some damage from trailering in the keel trunk which solved the mystery leak. After painting the keel trunk with bottom paint, I then installed the new keel line, and reinstalled the keel in about 20 mins.
That said, our boat does have a access hole in the port side of the keel trunk that someone prior to us installed. I epoxied the edges of the opening sealing them properly and then covered the hole with a piece of 1/4" plexi glass. I was able to replace the keel line with it on the trailer through this hole but would not recommend doing that as it was a real PIA. It was actually easier and better to drop the keel out of the boat and do it right.
I had West Marine Rigging make our ne keel line from a poly line with a stainless thimble. I think it was maybe 30 bucks plus shipping.
You can check out our blog at
http://adventuresofthememorymakers.blogspot.com/ go back a few pages to the boat restoration part and you can see some pics of what you should see when you get into it. The keel is 400 lbs and a bit awkward to handle. I think Russ with Lady Loraine was able to just lift the boat up on the trailer and let the keel rest on the keel board. There will still be 6 to 8 inches of the keel up in the trunk when the attach point for the cable becomes visible. Just be careful and good luck,
Sam in Seymour