As far as I know, it is not possible to create the whole control on the client. But perhaps, you can add the RadTextBox to the RadMenuItem template and hide/ show the textbox on client side.
I've thought about setting it at design time, then just flipping it on and off; I was hoping to prevent the 'option' from being visible at all if I wasn't going to give access to it. I'm a curious guy and often investigate the source of sites I'm on, and I like to prevent people from seeing options I'm not giving them. hehe
So, preferably, I'd like to not even include that menu option.
It's probably going to boil down between a design time template or an implementation of RadWindow and aspx pages to do what I want.
I like the RadWindow option - The menu option just flowed better with the rest of what I'm doing.