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It would be almost identical to my Client-Side Validated TextField, minus the validation. It captures input in onKeyUp() and displays the validation error in a <span> element after the text field.
See the project site for a demo and a link to the source code. It's a fairly simple extension of the TextField component. The most relevant source file would be VCSValidatedTextField.java. You wouldn't need much server-client serialization in a component with just a counter, unless you want to have some server-side API for customizing the counter (would be very useful to have reverse counter at least).