Couple things that come to mind:
- You'd need permission from your state to defer your service. I've always heard that they would be willing to do so for active Army service, but I can't verify.
- When you switch to active Army voluntarily, I've been told it's a minimum two year hitch. I'm not sure if that would include time in fellowship or only afterwards.
- Unless you volunteered for active prior to applying to fellowship, my understanding is that Reserve Corps applicants are considered after the placement of applicants from within active service. So if you were gunning for a popular specialty in which there were more applicants than slots, you might be SOL. I know that was the policy for military residencies, maybe they have a different one for fellowships, but I'd be surprised.