I'm not sure how many people I could find without a sternum, but the whole point of compressions is to push the blood out of the heart. Not compressing as deep in this situation would be the same as not compressing as deep in someone with a sternum, there wouldn't be as much blood flow.
I probably explained that poorly. In either case, you're compressing the mushy part in the middle, you're not really compressing the sternum itself (okay, you're compressing it a veeeeery small amount), so you're compressing the heart an inch and a half in either scenario.
I would say they are screwed. Sternotomy patients have just a flap of skin over the myocardium. So direct pressure will be placed upon the heart itself, I would assume lower pressure ratio as you described would be encouraged. The best way to gage would be to check perfusion levels per carotid pulse.
I have performed CPR on that type of patient as well as most post CABG (by-pass) patients have had sternectomy (albeit not removed) but cut through and wired.