Actually, I have a possible solution for that... But I have not tried it.
On the sky box itself, there is a 2 RF output, one for the main TV, and one for a second room. In the SKY menu settings, you have a format setting for the main room and another one for the 2nd room.
I would try the following:
Set the main room to 16:9 and the 2nd room to 4:3.
I would connect the Areal to the sky box, the sky box to the TV by the Scart plug and the RF cable.
On the link room TV, I would tune the LINK DTV frequency to the frequency of the Skybox rather than the BV3.
If the SKY box is the only video source that is accessed from link room, you can connect the RF output of the sky box directly to the MX4200 for a better picture quality.
Not sure if that's clear... but quite sure it would work ;-)