Ok, here is a question which probably has been discussed to death
already but which I would not know due to being a newcomer on this
list:

Has anybody of the LaTeX3 team yet taken a look at the ltxgrid package
by Arthur Ogawa?  While it does not have the figure placement
folderol of xor, it does offer nice ways of extending output
routines, both in the area of the actual code as well as magical
penalties (<-20000), can make longtable play together with multiple
columns and so on.

It does not do what xor does out of the box, but it offers much nicer
hooks for playing together with others.  The magical penalty stuff is
basically what I had been proposing, only that there is missing an
allocation macro for the same, and that there is missing a _context_
for the same, since one might want to have one and the same penalty
behave differently whether one is in an output routine for a
particular column, or just rushing a footnote box through penalty
processing in order to get margine notes and so on.

The stuff is used in RevTeX4.

It is probably not a bad idea to skim for some ideas...  although you
probably have done so already.

--
David Kastrup, Kriemhildstr. 15, 44793 Bochum