What my tests show is that it's not possible to use system property expressions for defining attributes in web administration console.

When I tried to set attribute (e.g. "Stateful Access Timeout" - Profile/Containter/EJB3) like

${some.variable:5000}

the console does not show any error message but it does not show the information about successful update. After "Save" it just takes the expression value and says nothing.
The change is not propagated (not saved) to xml config file.