I think it would a great idea to plug in to a number of such services, ideally without having to touch any code.
Consider only the case of entity recognition for the moment (not including relationship mining).
Here's a sketch of an idea. Generally, these services are REST based, and pass the text as some sort of post. They
reply with some XML format that reports the entities, position, and some metadata.
Suppose we can massage the XML that comes back into some relatively simple format, XML-based too, using either languages like XSLT, Xquery, or even a scripting language.
The plugin eats that simpler format.
So adding a new one is a matter of a configuration file that specifies the URI of the service, a template for the post, and a script to massage the results into the simpler format.
I can fill in more details, but that's the idea.
The issue I see currently is that I don't think the underlying smart tags engine can handle overlapping markup. Because of the state of the art of such systems it is likely that there will be overlapping (incorrect) tagging and that has to be present at
least long enough to be able to say which annotation is right and which is wrong.