An impressive question type. It's gratifying to see people using the most advanced features of CodeRunner.
I can't think of a nice solution to your problem, but an ugly one that might work for you is to make use of the 'partially correct' background colouring. This is used whenever the all_or_nothing checkbox is unchecked and the fraction is in the range 0.00002 and 0.99998 (roughly). So when in exam mode you could return marks in that range without any information leakage and with negligible actual impact on marks, which are always displayed rounded in Moodle, AFAIK.
Alternatively the way I dealt with a somewhat similar problem myself was to have the Check button simply print a message saying that Check functionality was disabled during the exam. The Precheck button still worked as usual, so they could still sanity check their answers. Then after the exam was over I switched grading on again and regraded all submissions. The regrading can take a long time, though, as all prechecks are regraded too.