(greggman) SlickEdit is supposed to recognize the:
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
in HTML files. Go to Tools>Options>File Extension Setup>General Tab and make sure the encoding is set to "Auto HTML". If there is still a problem, post another forum message and attach the HTML file. SlickEdit does have a couple limitations with this feature. SlickEdit uses only the first HTML encoding definition. Also, SlickEdit only looks at the first 4k of the file.
As for detecting code page files like euc-jp and shift-jis, SlickEdit does not have this yet. Microsoft word seems to do this.
There isn't a hook to detect encoding. However, you can modify the "build_load_options" macro. You could make a copy of this function, modify it, and load your own version. That way you would never lose your changes. The last macro loaded with this function wins :-)