Disclaimer: I'm not an expert on this at all! With that out of the way ...
AFAIK generally speaking there are many ways to attach messages to transactions, but most are considered hacks, have been disabled, are simply not supported by any client or rely on thrid party services. But there is one official way I happened to read about a while ago
linked here. This was added to Bitcoin core almost exactly one year ago, so I
believe Unobtanium should have that capability.
Basically what it does is OP_RETURN allows you to attach up to 80 bytes of data to a transaction. This is not exposed in the client though, you need to use the transaction API directly to read/write those. Maybe Blazr can give you some more information on this topic, wether it is supported in Unobtanium and if there are any plans to expose this functionality to Unobtanium users...