If each module is a separate file, which will be required to 'sell/deploy' each module separately, then you could host the new data file and distribute the new Front End to take support it.
Each version of the Front End file will only be used with the specific set of modules it supports. It will take a lot of work to make a universal Front End that doesn't constantly ask for the missing data files.
You should consider external authentication, where FileMaker relies on an Active Directory or Open Directory server to handle authentication. This way, FileMaker files aren't tracking user account names and passwords internally, which makes it easier to swap out old files for new ones.
I had thought about external authentication, but I need the ability to allow a user to change their own passwords and I also need the ability for an area manager to create a userid for an entry level employee, which I can script when I use the Filemaker users.
For one of our solutions, we used one file just for Login, that set Globals, then every other file used it to check for "permissions"
so long as you don't need to lock the data itself
> I need the ability to allow a user to change their own passwords and I also need the ability for an area manager to create a userid for an entry level employee