Hi,
If you're running from within the gfsh shell, you should be able to enclose the argument to --J with single quotes and have it work:
start locator --name=locator --port=19991 --J='-Da=foo,b,c,d'
From the command line you need to escape the quotes:
gfsh start locator --name=locator --port=19991 --J=\'-Da=foo,b,c,d\'
--Jens