Could the info needed be compressed to fit into 40 bytes?
What sort of entropy does the typical XCP OP_RETURN data have?

Depending on the data, it might be possible to create a small customized compress/decompress function

If someone can post an example it would help me analyze this a bit more

Edit: due to the importance of this I think it would even make sense to create a custom "dictionary" of common bit patterns (hopefully there are some) in the desired OP_RETURN data. This would have to be put into the counterpartyd, but doing stuff like this might be what is needed to get 2:1 compression, then again maybe standard compression will just work, but I think some sort of custom code would have a good chance. It all depends on the type of data.

James