Shaders are an option. Only performing rendering operations a defined range of UV coordinates. Could you explain more on what you are trying to accomplish?

Edit: Double posting is bad *shakes finger*

if you are speaking about render target, you can set a viewport and then render.

Thanks everyone, after some testing I went with rendering a smaller quad against the large texture, works great.

Also, I didn't intentionally double-post, I initially posted it and gamedev reported an error twice. After I checked to see if it had gone through despite the error [both times] and it hadn't, I tried again. I'm a regular on this site with a lot of posts so I can't really understand why anyone would assume I had double-posted intentionally.

I also received a warning about it from a mod - not appreciated at all after contributing to the site financially and it being gamedev's fault. *shakes head faster than finger*