Summer, if you haven't already, I recommend reviewing LEED's Minimum Program Requirements for background on some rules relating to this issue. Other reading material that may be useful is the LEED application guide for multiple buildings and campuses. Note, however, that this was applicable to LEED-NC v2.2 and has not been update for LEED 2009.
But to answer your question—my quick take would be that you can exclude the planned building from your LEED boundary, as long as the planned buildnig is detached, and that you don't have to gerrymander your boundary to accomplish this.