jrash06 It depends on the particular use case...
For example: Who is going to message who, e.g. is it the client who sends out messages to customers (e.g. one user sends messages to many) or users send messages to users? -> Not 100% sure, but afaik you cannot send messages as another user trough the Lithium API, e.g. playing man-in-the-middle between two users chatting is most likely not going to work.
In general: the messaging API (or the PM module in general) is pretty rudimentary, which is why I would not build custom apps around it. Plus: Lithium is working on a new PM module that is supposed to come out this year, that would also mean, that you might have to rewrite your entire Lithium side code, once this is out or you will be stuck with the primitive old API if you don't.
If the chats are managed on an external service anyways, it might be easier to entirely replace the Lithium PM module with your own customization, e.g. build a custom UI on Lithium, that interacts trough the http.client with your external services (e.g. fetching PMs, displaying them on Lithiums side in the PM area of the user profile, having some custom forms that are sending the messages to your service etc.). Depending on the implementation a potential challenge could be how you link your chat services users with the Lithium community ones, if it is only dealing with users from Lithium communities that wouldn't be a big deal, but if the service deals with its own user-base, it could be, but as said before, that depends on the implementation. Feel free to contact me if you need professional help with that customization (also "just" the implementation on Lithiums side). Stuff like that is what we do best =)...
