You absolutely can control a projector through a US Avant. Using the "Option 0" command on the Beo 4 will turn off the Avant's screen and transfer the video signal to a connected projector. In fact, you can even have the Avant change the center channel to an external center channel speaker when using the projector. This feature was added in case the projector's screen dropped down to cover the Avant's built-in speaker. All of the other surround speakers will still be used.
I have a Runco projector connected to my Avant in the master bedroom. Using the projector, I can have the video image visible from a jacuzzi in the bedroom. Makes for a very nice cinema experience - especially with champagne!
You are rather limited in the projectors you can use. I know that both the Runco and the Sim2 HT250 are perfectly compatible. Only 1080i is compatible (not 1080p) and you can find some pretty good deals on used Runco and Sim2 projectors on eBay.
Something to keep in mind is that the Avant's internal video is 480i or 480p and not 1080i. However, when the signal is sent to the projector, the image IS 1080i from the projector. In many cases, because of the Avant's video processing, the 480i signal on the Avant is actually superior to the 480p signal. Therefore, it is actually best to use TWO connections when connecting, for example, a blu-ray player to the Avant. You want to use a component cable connection to the Avant (or to a Zektor switch box connected to the Avant - the Avant can control the Zektor switch box if the software and computer chips are adequately updated) and also use an s-video connection to the Avant. Set the blu-ray or other device to 1080i output. By doing this, the Avant will send a 1080i signal to the projector and a 480i signal to the Avant's screen. This works beautifully.