Why, oh why do people insist on hiding their cabling without future proofing it - easy with a small amount of thought or asking a question or two here first.
The Anytime functions would only work in a very limited way and you would only receive less than half the available Anytime downloads. For full functionality of Sky+ or HD you are going to have to bite the bullet and fit the extra cable. You will be extremely unlikely to get a deal from Sky on this as Sky installers can and probably will refuse to install without the extra cabling and the ability to inspect the current cabling to check for damage to the cable which possibly could damage the box they install - they after all are offering a guarantee on the complete installation. The only way you will probably get the installation you want on one cable is to buy the box elsewhere and DIY OR go through a local independant installer. Sky will only do a standard install and this means two cables NOT just one.