It seems to me that relying on internal hostdomain settings to get access to the right functions for the tool is fraught with danger: all it takes is for one user to know that this is how you enable a given featureset, and if he can figure out the hostdomain for a company that's paid you more $$$ to get more features activated, he may then spoof being from that other company, and get extra features for free.

I would think a better way would be to give each company unique license files, and have the application transmit some identifier based on the license file when it's talking with the server (maybe using Public Key Authentication), and have the server only activate the features when given a license key that has those features available.