How much salt can I safely add to bread dough?
Based on experience and experimentation alone (I bake bread several times a week), I'd say to be consistently safe you probably want to max out around 3% in baker's percentages.  2-2.5% is much more common, but I've done as much as 3%.  After 3%, things got very inconsistent.  Sometimes it would work if the structure was just right, but mostly I got loaves that didn't rise well.

If I did my math and conversions to weight right, it looks like you're probably in the 2% range, so you should be able to add about 50% more salt.

EDIT:  It must be higher, but I'm not sure how high.  Bread Baker's Apprentice as a 4.2% Poolish Focaccia and other others in that range.  BBA seems to max out in the low 4's.