Yes you can 'bypass' the stereo proc.
As explained in : http://wiki.ros.org/stereo_image_proc...
This package contains nodelets for creating disparity images and point clouds from stereo. This allows stereo_image_proc functions to be easily combined with other nodelets, for example camera drivers or higher-level vision processing. In fact, the stereo_image_proc node simply loads a combination of nodelets.
To get the individual functionalities from stereo proc you need to use it's nodelets. As of now it has three:
Disparity, Pointcloud, and Pointcloud2.
Using nodelets allows you to compose your own image pipe line. See: http://wiki.ros.org/nodelet
For example, this launch file connects a few nodelets together. Currently there are only pre-cooked nodelets. But we plan to add custom nodelets into this pipeline.
<node pkg="nodelet" type="nodelet" name="nodelet_manager" args="manager"/>
<node pkg="nodelet" type="nodelet" name="left_rectifier"
args="load image_proc/rectify nodelet_manager">
<remap from="/image_mono" to="/camera/left/image_raw"/>
<remap from="/camera_info" to="/camera/left/camera_info"/>
<remap from="/image_rect" to="/camera/left/image_rect"/>
</node>
<node pkg="nodelet" type="nodelet" name="right_rectifier"
args="load image_proc/rectify nodelet_manager">
<remap from="/image_mono" to="/camera/right/image_raw"/>
<remap from="/camera_info" to="/camera/right/camera_info"/>
<remap from="/image_rect" to="/camera/right/image_rect"/>
</node>
<node pkg="nodelet" type="nodelet" name="disparity_gen"
args="load stereo_image_proc/disparity nodelet_manager">
<param name="approximate_sync" value="true"/>
<remap from="/left/image_rect" to="/camera/left/image_rect"/>
<remap from="/left/camera_info" to="/camera/left/camera_info"/>
<remap from="/right/image_rect" to="/camera/right/image_rect"/>
<remap from="/right/camera_info" to="/camera/right/camera_info"/>
<remap from="/disparity" to="/camera/disparity"/>
</node>