The commented backslash hack.

This is a simple method of targetting rules specifically at just macIE5. It relies on the curious fact that macIE5 apparently gags on a backslash in a /* comment */ and forgets to parse the following rule. Since putting this test page together I've since found that:
The comment must be inbetween rules, not within a declaration block {}.
If the following rule's selector is an element (e.g body), the hack fails, it needs to be a . or #. (I suspect the significance of this is that class and id selectors start with non-alphanumeric characters, and for some reason the parser fails to ignore the commented backslash and tries to escape the next uncommented character it finds -- resulting in an invalid rule)
Whatever the cause, this opens the door to working around that irritating affection macIE5 has with horizontal scrollbars...

MacIE5, overflow: auto and padding-right.

This goes some way towards a viable workaround. A couple of issues remain open, but there are no horizontal scrollbars, and other browsers are unnaffected - by my testing at least.