In an earlier article I noted that Unicode 5.1 added codepoints for Ideographic Variation Sequences (IVS), which can be used to trigger display of glyph variants in Chinese, Japanese, and Korean.
OS X 10.6 has support for IVS in TextEdit, as described in this recent blog article. All you need is one of the Adobe Pr6N Japanese fonts.
Inputting an IVS is not so simple, however, because Character Viewer won't scroll down to U+E0100 and the Unicode Hex keyboard requires typing in two surrogates. The easiest way is to have UnicodeChecker on your machine. Then you can copy/paste from that or type in the character entity (like 󠄀 ) and convert it via the html > unicode item in the Services menu.
PS I see now an easier way. You can put the IVS characters in the Kotoeri dictionary and invoke them via vs0, vs1, etc. directly from the Kotoeri IM. Here is a demo page I did using that method.