The following mostly works. 'Mostly', because the use of the SOMETHING..\tasks\ pathname confuses Spring when a context XML file tries to include another by relative pathname. So, what I seem to need is a way, in a BAT file, of setting a variable to the parent directory of a pathname.
set ROOT=%~dp0
java -Xmx1g -jar %ROOT%\..\lib\ajar.jar %ROOT%\..\tasks\fas-model.xml tasks