Strangely, there doesn't seem to be any way to prompt the user for anything more than a boolean. I suppose your script could play 20 Questions using user_confirm to input arbitrary data...
I misread your question at first, and thought you were asking for a way to pop up one of the existing value selecting dialogs at an arbitrary time. I did come up with a way to do that; it's ugly, but you'd at least be able to prompt for a string or integer (perhaps to select from a list previously printed to the gCLI window). The idea is to create helper scripts, one for each type of data you need to input, with a main() function that takes one parameter of that type. All they would do is store that parameter in a property, "temp" perhaps. In your main script, you'd do a cli_execute() of the helper script, passing no parameters (so that the automatic prompting kicks in), and then examine the property.