Also certain adjustments you want to make on the fly where you want to see the HU might not be so easy.
When alarms go off what do you see anybody do about it? That is how useful they are. A better idea I've seen was a guy on a Tundra forum mocked up a dummy stock JBL faceplate for his aftermarket double din. I myself actually ran tape decks with remote changers until only a couple years ago. Last setup was Alpine. The truck was actually broken into once and they ignored it. The faceplate was on and everything, but they ignored the head unit and dont appear to have even looked for or tried to remove the 6 disk changer. They only went for some stuff I foolishly left sitting in plain view on the backseat. Now this was in plain view in a busy street in a downtown area on a Saturday night when everybody was out.
In college they had a rash of smash and grabs on campus. I had the same setup, tape deck with remote 12 disk changer, only it was JVC. Girlfriend always ragged on me for being in the stone age and having a tape deck. Well her car was near mine, both faceplates on, hers a single CD, mine a tape deck with remote changer. They broke a window and took hers and left my truck alone.
I have single CD now because it's what you have to do to get things like IPOD connectivity, bluetooth, and I don't really listen to tapes anymore. But what Id really like to see is a real good SQ oriented CD head unit with all the latest features but a plain jane tape deck face plate that flips out to reveal the CD slot behind it. If they want it they'll get it. Your only real chance is to make them not think it's worth it.