Directionality in Math Zones

In most places, mathematical text is written “left to right” (LTR). For example, in the expression x + y the plus is displayed to the right of the x and the y is displayed to the right of the plus. But in some Arabic locales, mathematical text is written right to left (RTL). Instead of E = mc2, one would see 2cm = E, although the letters would be Arabic, not Latin. In such RTL locales, square roots are mirrored, so that the surd symbol √ is flipped relative to the vertical axis. Similarly integral signs are mirrored, although the circular arrows in contour integrals are not mirrored, since they pertain to the 2D complex plane, not the 2D text plane.
The Presentation MathML 3.0 specification provides for RTL math zones. In fact, it allows a dir = “ltr” or “rtl” attribute on the top level <math> element as well as on <mrow>, <mstyle> and token elements like <mi>. Except in rare cases, only the <math> direction need be specified, since all the elements inside have the same directionality (see Section 3.15 of the MathML 3.0 specification). The specification has now undergone Last Call status and so we need to have implementations of the new features. Accordingly I’m interested in implementing at least part of the RTL functionality, namely RTL math zones.
First, consider what an LTR math zone is. This is what Word 2007 and the Office 2010 applications implement. It does have RTL text whenever Arabic or standard Hebrew characters appear adjacent to one another. But all operators and other “neutral” characters are considered to be “strong LTR”, that is, they are displayed to the right of the character that precedes them. This can be quite different from a display that obeys the Unicode Bidirectional Algorithm. A sequence of digits