Support for a normalized Unicode string is required.

According to MSDN -
Some Unicode characters have multiple equivalent binary representations consisting of sets of combining and/or composite Unicode characters. The Unicode standard defines a process called normalization that returns one binary representation when given any of the equivalent binary representations of a character. Normalization can be performed with several algorithms, called normalization forms, that obey different rules, as described in Using Unicode Normalization to Represent Strings. The Win32 and the .NET Framework currently support normalization forms C, D, KC, and KD, as defined in Unicode Standard Annex #15: Unicode Normalization Forms. Normalized strings are typically evaluated with an ordinal comparison.

According to CCTS
Normalized string is a string that does not contain the carriage return (#xD), line feed(#xA) nor tab (#x9) characters.

From http://unicode.org/reports/tr15/
The W3C Character Model for the World Wide Web, Part II: Normalization [CharNorm] and other W3C Specifications (such as XML 1.0 5th Edition) recommend using Normalization Form C for all content, because this form avoids potential interoperability problems arising from the use of canonically equivalent, yet different, character sequences in document formats on the Web. See the W3C Requirements for String Identity, Matching, and String Indexing [CharReq] for more background.

The normalization standards have nothing to do with control characters – this standard describes them http://unicode.org/reports/tr29/

OPC UA defines a String to be:
Unicode character string that should exclude control characters that are not whitespaces (0x00 - 0x08, 0x0E-0x1F or 0x7F).

Although this definition does not seem to include all of the possible characters and probably requires a seperate mantis issue (or it code be fixed under this one, since it would be just a text clarification not a functional change)

Conclusion – we can pick one of the normalized formats (Form C probably) and make a subtype of string that is NormalizedString, since string already handles the control characters – this is assuming that the list of control characters is expanded for string.