There is not only a "script" value, "rel" can contain any valid attribute content. The questions are:
- Which values have meanings defined (usually via specifications like XHTML 1.0)?
- Which browsers support those?
Obviously, the W3C wanted scripts to be includable like stylesheets (<link rel="Stylesheets" …>
), but did not mention it in the prosa sections of the spec, only in the DTD comments. You find it in the HTML 4.01 strict DTD as well (XHTML 1.0 is just a reformulation of HTML 4.01 in XML). Strangely, it is neither not mentioned in the corresponding link type section nor in the script section. It could be an artifact of an idea which was rejected later.
This brings us to the answer to question 2. It is not properly implemented in browsers, at least in Safari (you mentioned it yourself). Therefore, you should not use it, because it will not work cross-browser. You can use it as a hack (to include scripts which will not be loaded by Safari), but this is not safe, because you cannot tell if a later version of Safari changes the behaviour.