How can Finder permanently show a specific file starting with a dot/period?
One way to get around this would be to create a symbolic link to .htaccess and call it something without a dot. For example:

$ cd site_root
$ ln -s .htaccess htaccess


Then you can open htaccess from Finder and you'll actually be editing the .htaccess file.

(You could even make the second command ln -s .htaccess " .htaccess", which would visually look like .htaccess but be visible because of the leading space.)