One method would be to use spray coating to deposit resist directly to
the sidewall of the trench. You can find details at
http://www.micromagazine.com/archive/04/03/brubaker.html.

Another method would be to use a very thick layer of thick resist(AZ9260
or SPR220 7.0 as a minimum), and nearly fill the trench.  The problem
with this is that it would also leave a thick layer of resist elsewhere
on the wafer, adding complexity to the lithography process (and limiting
the minimum resolution available).

Best Regards,
Chad Brubaker