read,
Welcome to Apple discussions.
If you're using Pages 3, you posted in the wrong group. This is the pre-iWork '08 group. But we'll forgive you.
I can understand why you'd want a text box that's locked in position but is still editable -- but I don't think it's possible using the conventional method. When you lock an object with Arrange > Lock, not only is its position locked, but you're prevented from clicking inside of it, preventing you from editing the text.
That's the bad news. The slightly good news is that there's a kludge I came up with that might help. Use a combination of blank spaces and Returns to position it on the page, then redefine the text box as Inline instead of Floating (see the Wrap inspector). This will fix the box on the page (unless your users add more spaces or Returns), yet allow editing of the text. You just may have to fiddle with its position to get it where you want it. But at least your users won't be able to drag it away.
I'm confused by your second question. Are you trying to have placeholder text and locked text inside the same text box? If so, I don't think it's possible.
-Dennis