SharePoint is really nothing more than a bit of a content platform that runs off a SQL Server backend. I'm oversimplifying of course.
Given that everything gets stored through the SQL server theoretically you can use ESS. The issue is more: does IT let you do it?
SharePoint developers typically use a .NET based language, which can talk through FMS through ODBC/JDBC and XML. See also www.fmdotnet.org
From a purely technical point of view there is a lot of integration that can be done. Politics is usually the bigger problem.
If you're looking for a technical answer, I'm afraid I'm unable to help. From a SharePoint perspective though, there is a multiplatform option available to hook into called CMIS which is an international standard, currently managed by OASIS.
Microsoft doesn't even believe that SharePoint is the be-all and end all of CMIS systems so how is it valid that some corporate IT team knows better? The problem with this approach is that it is unclear to me whether anybody can get FM accepted by the CMIS borg in any meaningful way. If it can, you can probably continue to slip FM into SharePoint shops going forward so long as you do it in a CMIS compliant fashion.
display FMp data in Sharepoint via html/xml or a custom PHP page for interactivity with the data works