You can create your template within logjam, and then do:
Entry -> Save As -> File...
and save it as, for instance, ~/.logjam/template .

If you then launch logjam with -f ~/.logjam/template it'll always start with your template entered in. It's not a 100% solution as after submitting you'll need to manually load it again. If you generally only post one entry each time you open logjam, though, it should get the job done.