[XeTeX] Overriding Unicode text direction with \beginR ... \endR

[David M. Jones]

When I typeset the following document with r803 of the XeTeX trunk,
the first row of digits (in CMR) is reversed, but the second isn't.

    \TeXXeTstate=1
    
    \nopagenumbers
    
    \beginR
    0123456789
    \endR
    
    \font\0="Gentium"
    \0
    
    \beginR
    0123456789
    \endR
    
    \bye

I assume this is because in the second case, the Unicode
directionality of the Basic Latin plane is overriding \beginR.  Is
there a way to override the overriding?

Cheers,
David.