I'm not an expert, but I think you absolutely should approach them. When you understand the rationality behind acquisitions you can see why I'd say that. Take Google for example, who has been on a tear with acquisitions, along with others. Look at their structure, at their company direction. Acquisitions for them make sense for several reasons: access to the brainpower of the startup, consolidation of power, more favorable reviews in the community. All that and it costs peanuts from their perspective.
Contrast that with the publisher of D&D. How forward thinking, in terms of having strategic technological positioning, is their management? Probably not as much as Google. All they are concerned about is a 1 to 1 correlation with profits. Action A yields profit B. If they see that they can build your functionality in house, they may see a lot of advantages to that. They don't care about the other stuff as Google might. I'd come up with a figure to be happy with, yet highly enticing to them, and then sell it with rationality. Point out all the benefits, of which access to you should be the biggest selling point, as you've successfully built it up.
Contrast that with the publisher of D&D. How forward thinking, in terms of having strategic technological positioning, is their management? Probably not as much as Google. All they are concerned about is a 1 to 1 correlation with profits. Action A yields profit B. If they see that they can build your functionality in house, they may see a lot of advantages to that. They don't care about the other stuff as Google might. I'd come up with a figure to be happy with, yet highly enticing to them, and then sell it with rationality. Point out all the benefits, of which access to you should be the biggest selling point, as you've successfully built it up.