yeah... i work in an emergency room so i have a familiar knowledge of radiology and of neuro stuff like EEGs.
smoking weed would not affect the MRI at all... guaranteed.
for the EEG i would actually encourage you to maintain a normal life pattern. if that includes smoking weed, then smoke weed. the EEG records brain patterns and activity so weed could only effect the test in as much as it effects your brain waves. if that is a normal chemical in your brain then keep it in there for the most accurate diagnostic.
disclaimer: i am not a physician. i would consult your physician for authoritative information on this topic. this is my non-professional opinion. call your doctor up and say you have an important yet sensitive issue to mention before the test. be up front and see what he says.
"keep on living, keep on loving, keep on smoking" ~cody chesnutt