The only way to get south of Blackwater that I know of and not be hunted, is to play the chapter 3 mission, I think it is chapter 3, where you go to Bronte's mansion to get JAck back, in that mission one of the tasks you have to complete is search the cemetery and not be seen, But if at some point you are seen by police, yet you still escape and live, to finish the mission, afterwards when it is all over, for some reason, you can then travel to Blackwater or any other part of New Austin and nobody will hassle you.
Now I read that this was patched, so I am not sure if you can still do this or not.
When I did it, it worked perfectly.
UPDATE:
So once I got to the epilogue and was able to free roam, I had some mail, and it was the invite from Gil, which is odd, since John never met Gil, or sent him any fish.
anyway, the story went on as it should and left me as John at Rio Bravo.
SO I guess R* never recorded script from Arthur in case anyone did this, and the game is a bit linear in that respect as far as this fishing legendary fish is concerned.
Im sure even though they had the budget and time allowances they had, that they still woudl have loved more time and more money, the game as awesome as it is, has many areas where I think hmm it woudl be nice if... and then I think, I bet they wanted that, but..... I cant wait to see what the future holds.