By script do you mean truly a script file (*.scr) or are you using lisp or VBA?
There are lisp functions that can be used to handle the rendering, without a dialogue box. The the lisp reference, commands are "render" "rfileopt" (for rendering to a file), "rmat", etc.
You can include a lisp command in your script, such as (from the lisp reference):
(C:RFILEOPT "PCX" 640 480 1.0 "G8")