[XeTeX] Xetex equiv to luatex's \directlua{}

[maxwell]

I'm just finishing up a project that involved typesetting text in 
several languages, while outputting an XML file that defined in X/Y 
coordinates the position and size of the bounding box surrounding each 
line of text in the PDF.  I used Luatex, because that made it possible 
to call Lua from Luatex using the \directlua{} command) to pass 
information to Lua, and to return information from Lua to Luatex using 
tex.print().  I also used Lua to write the XML file.

Too late, I discovered that LuaTeX botches the rendering of one of the 
languages, Tamil.  Tamil has a complex script, with some typical Indic 
script features; so presumably LuaTeX would also mess up on other 
languages with complex scripts.  XeTeX of course does just fine at 
rendering text in complex scripts.

As I say, it's too late to change now, but is there any way I could have 
done something similar using xetex?  That is, called another programming 
language to output box positions and sizes.  I suppose it's possible to 
write to an XML file in xetex natively, but I'm not sure how I could get 
the positions and sizes of boxes.  My style sheet defines a command, 
\outputpara{}, that requires the user to specify the X-position of the 
paragraph, and hence of lines in the paragraph, where line breaks are of 
course decided on the fly.  The command optionally specifies the 
Y-position of the paragraph, but the Y-position of each line in the 
paragraph--except the first--is determined by the usual TeX algorithms.  
Getting TeX to tell me those Y-positions, as well as the vertical size 
of the box, was the difficult part.  But maybe I was missing something 
obvious?

    Mike Maxwell
    University of Maryland