LO already has some other security features
I must admit I haven't experimented with the digital signatures option, but the already integrated TSCP compliant facility to classify documents is of interest for us, although it requires more study before we decide if we implement it (we will, for instance, need to test if we can set up pre-classified templates). Apparently it's implemented to the point of refusing to copy classified information to a document classed as less secure.
Now, keep in mind that this is *free* software. Amazing. If it wasn't for our Excel munchers I don't think we'd use MS Office at all.