Given iPhone is an Apple product and OneDrive is a Microsoft one I would not be too hopeful of there every being support between these two.
The point is somewhat mute though. A free Dropbox account is more than sufficient for Moneydance. no matter how many devices you have. You get three client licenses with the free account yet you need not use any.
The Moneydance mobiles app will never use a Dropbox license as it uses the Dropbox API and not the Dropbox client - it is the client that meters the license limit, so a license will only be used if you were to install the client.
The Moneydance desktop software only requires a license if you choose the syncing method Dropbox Folder which in turn requires the client software. Dropbox Connection makes use of the API again and in itself does not require a client license.