If the customer refuses to spend the money to update the whole system properly, then as always, you are left with 2 choices.

1) Drop the customer. If you don't need the work and don't want your name attached to the whole thing (because customers always blame everything on the last person to touch the system). Then just decline to do anything but a full upgrade.

2) As mentioned above, cobble something together.
Mac may not be aware but Roku still makes the Express+ : [Link: roku.com]
$30 each TV. No need for an HDMI adapter, just plug the A/V cables into the old TVs. Then a quick program change on each MX900 as suggested above (no need to mess with the MSC400) and they are done.