Obviously there are times were such an act is in fact ethical. Bipartisan politics often involve give and take when things need to be accomplished.

I would argue that in picking the President of the United States(or in this case the nominee) that the race be as democratically “pure” as possible. Mr. Obama and Mrs. Clinton both ran solid campaigns, but the people voted for one or the other. They did not vote for Mr. Obama, with Mrs. Clinton picking a Supreme Court justice, and other judicial appointments. The problem here is that is an inherently corruptive force. While you put this forward with the best intentions, there is a serious danger in blurring the lines of what is ethical and what is not ethical in compromise and party politics. I suppose if you made the deal explicitly clear to the voters well before November and gave full disclosure of what would happen, then it would be…..tolerable.
Even worse, nothing has been decided until the Democratic Convention. Until the delegates cast their votes, Mr. Obama is not the official nominee. Mrs. Clinton may be up to her tired old scheming so your points may not be as moot as many would like them to be.