You sometimes hear, most recently from Ezra Klein, that it would be illegal to appoint a shadow cabinet during the campaign because of the law against “directly or indirectly” promising anyone government jobs. In practice, though, it’s easy enough to leak that you’d appoint “someone like Chuck Hagel” as Secretary of Defense if you want to signal that you’ll appoint Chuck Hagel as Secretary of Defense. The reason not to do that isn’t the legal problem, it’s the other consideration Ezra brings to bear — you don’t want to re-enforce the idea that progressives can’t handle the job. If you’re going to go the route of putting a politician in the Secretary’s chair (which, paired with a Deputy Secretary from the policy world, seems pretty likely) then pick a progressive politician.
It would be much more productive, I think, to take someone with a solidly conservative domestic record but internationalist views on foreign policy and make him (or her) UN Ambassador or something. That sends the message that the liberal approach to world affairs has appeal that transcends party lines or debates over tax policy or whatever else. Or, similarly, if you could find someone with a generally conservative record but sound views on climate change and give that person an environmental policy role. Those are ways of co-opting conservative politicians in order to broaden the appeal of progressive solutions, rather than a way that draws attention to alleged weaknesses in the progressive approach.