It's certainly possible, but you will have to determine exactly what encodings you're targeting. The encode-characters proc and method would probably be of use to you here. You could convert the String value of the TextField to a ByteVec and see how long that is. I recommend looking at the documentation for encode-characters and CharEncoding. There's an example in "Encoding Strings of Characters" in the documentation chapter "Reading and Writing Text Files".

For the best implementation, you would want to override the TextFieldUI you were using, probably just to check every time an insertable KeyPress happens, and possibly even the TextField itself. You could avoid some of that work if you were willing to just truncate the value in the TextField on an overflow, or something similar.

As a quick example, the following writes out the length in bytes for the given encoding on each ValueChanged. The target encoding is utf8 here, though you might be looking for "shift-jis" or "euc-jp".

{TextField
    {on ValueChanged at tf:TextField do
        def in = tf.value
        let unicodeout:ByteVec =
            {ByteVec
                max-size = {value in.size} *
                CharEncoding.utf8.transcode-max-expansion-factor
            }
        || Use get-character-encoding-by-name procedure
        let encoding:CharEncoding =
            {get-character-encoding-by-name  "utf8"}
        let unicodecount:int =
            {encode-characters in, unicodeout, encoding}
        {dump unicodeout.size}
    }
}