Thanks for the responses, guys
My plan is to transfer the recording to the US and watch them from there. I figured I'd set up the VPN directly between my core in the US and my device in the UK. Since I'm not too worried if the copy takes some time, this is preferable to watching directly over the VPN. I'm also looking at some kind of traffic-shaping system to ensure that the upload from the UK happens when my in-law's bandwidth isn't in use anyway (the middle of the night, for them!)
As I see it, I have two basic things I need to do, which I'm not sure how to or even if MCE can do!
1) Control a second Myth Back-end (the remote one). Whilst I know I can record on an MD, that would require my remote system PXE boot and be "inside" which is clearly not going to be the case!
2) When recordings are found remotely, copy them to the main storeage, rather than simply indexing them and making them available.