There are some pro and cons for using a system as such. Obviously not being able to add global field like you just found is one. Sorting the ESS table is going to be an issue as FileMaker bring down all the records and then sort them on client. ( totally inefficient ). Instead you should be using presorted views and "mimicking" a sort per se.
When it comes to large data sets, integration, or forced limitations such as having to use outter joins, ESS may be a great option. However, if just for the sake of it then I would advise keeping it natively within FileMaker.
I just hit this roadblock myself
My ESS records are displayed in a portal, and need to be filtered by a Global field
the filter works, if a Global Value is entered, but if left blank, it will not show all
Nice to see you post something that doesnt pertain to corruption. lol.
Anyway, take a look at this demo. It should still work for ESS tables as well.