If you have either 2 x V6 converted boxes or a 360 and a mini box I doubt if they will install a third box which is why I suggested a manned installation just incase you got sent a stream box which can't be on the same account as 360 the stream is only available for broadband customers. (The offshore team seem to mix up the mini box and stream box).
You could always put some trunking in and a drawstring but if it's an external wall and you can get a third box which I doubt the cable could be split externally and run round the outside.
It might be worth just asking for a manned installation and see what the engineer says because if it's a stream box then it's not going anywhere, if they are prepared to add a mini box then he should be able to suggest the best solution that you agree to.
Edited to say that if you did have 3 boxes it would definitely need an engineer to check your signal levels and adjusted in the street cabinet and balance the signal between the boxes.