I have experimented with String obfuscation using simple techniques such as Base64 or XOR. From my experience Proguard is not able to de-obfuscate the manually obfuscated data, even with optimization enabled.
You could try to decompile your own sourcecode after exporting your project with Proguard enabled using tools like APKtool, Dex2jar, etc. Then you should see whether the String was de-obfuscated or not.