Writing escaped unicode

I'd like to write unicode strings using the "\u" escape syntax.  According to the documentation, print.default or encodeString will escape unicode using the \u convention.  In practice, I can't make it work.

b="Unicode character: \ufffd"
print.default(b)

[1] "Unicode character: ?"

encodeString(b)

[1] "Unicode character: ?"

I want to write the string back out in the same escape formatting as I read it in.  This is because I'm interfacing with some Ruby code that requires unicode to be in this escaped format.

Thanks in advance!