Hello,
I think the standard ASCII character set only consists of values in the range: 0 - 127. That is, it may not be a printable character. Fortunately that doesn't matter; what really matters is that you send the byte(s) you would like, which has or have the underlying value 128 and/or 129. To do this, all you have to do is define a byte with the value 128 or 129, and then type cast it to type string. This will define a character or characters with that ascii value, regardless of whether or not you can read it in a string (ie. regardless of whether or not it's printable).
In LabVIEW you can use the type casting function located in the All Functions -> Advanced -> Data Manipulation palette. Simply wire a numeric of type byte to the left input, a dummy string to the top input (you can actually leave it since it will be string by default... but for readability it's nice to know precisely what your type casting to :), and the output will be a string with ascii value defined by the input byte.
Again in LabVIEW, if you want to define more than one byte at a time and build a string consisting of characters defined by an array of bytes, you can use the Byte Array to String function, found in All Functions -> String -> String/Array/Path Conversion palette.
These functions allow you to build an arbitrary string, irrespective of whether the characters are printable or not!
I think those functions should do the trick for you!
Repost if you have any further questions!
Best Regards,
JLS