>>> So a new command and keystroke seems warranted.  How about...
>>> `M-g M-v'?   (The mnemonic is "goto visual line".)
>>
>>   C-x n g         go to narrowed line
>
> Perhaps both?  The keystroke makes sense in both contexts -- as a
> variation on `M-g M-g', and in the group of narrowing keystroke.

Yep, having both is a win-win situation.

Here is the patch that:

1. leaves the existing 'goto-line' completely backward-compatible
   (actually a small difference is that in a narrowed buffer it displays
    now the prompt "Goto absolute line:" instead of just "Goto line:")
2. adds two optional args RELATIVE and WIDEN to 'goto-line';
3. adds two new commands 'goto-line-absolute' and 'goto-line-relative':
3.1. 'goto-line-absolute' widens the buffer and doesn't narrow it back;
3.2. 'goto-line-relative' is bound in Info mode to `M-g M-g'.

If this is ok, then 'goto-line-relative' could be bound to
`M-g M-v' and `C-x n g'.