After a quick search, I couldn't find anything about MirrorSync and FileMaker Cloud. 

Is MirrorSync compatible with FM Cloud?

Found it in the MS documentation, yes with an asterisk.

Where did you search? Googling for "MirrorSync FileMaker Cloud" takes me to this headline: "MirrorSync 3.1 Released with FileMaker Cloud Compatibility - 360Works"

So the short answer is, yes, it works fine :-)

The long answer is, yes, it works, BUT you will need to run MirrorSync on an separate computer from your FileMaker Server, because FileMaker Cloud does not allow you to install 3rd party software other than plug-ins (MirrorSync is not a plug-in). If you would like, we can run MirrorSync for you on our shared server for $29/month.

Is it possible to set up a configuration with a server-to-server sync between a local Filemaker LAN server and Filemaker cloud server, thus having the iPads, iPhones, etc from within the LAN sync to the local server and all external ip addresses syncing to the cloud server? We struggle on VSAT with high data costs and low upload speeds (1-2 Mbps) but can get downloads speeds around 12 Mbps. My thought is to allow external users to grab a copy from the cloud whenever needed without slowing down the ISP connection to the LAN server (except as changes occur to the LAN server) and improve speeds for remote work on the server copies of the databases.

Not sure if it's feasible or the right approach

You may also want to consider using our professional FileMaker virtual dedicated server hosting services.  On one VM with us, you can host a full install of FileMaker Server (not limited like FileMaker Cloud), as well as your MirrorSync install.  Have a look at:

http://www.pointinspace.com/

and let me know if you have any questions after reviewing either here or off-list to jmay(at)pointinspace.com.

- John

Sam, yes, it's definitely possible to set up what you're talking about. FileMaker Cloud will work well in a server-to-server sync environment. Remember as I said in the initial post that you can't actually install MirrorSync on the FileMaker Cloud box, so you'll need to run it on some other computer (such as the one in your office).