TRACON Controller described a MVA incursion event during a very low altimeter period involving a descended below assigned altitude; the reporter acknowledging the aircraft was 1;000 FT off altitude when checking on the frequency but that fact was missed.

Narrative

A G5 checked in from the east inbound. Our altimeter was very low. I have started using 90 on initial descent. Aircraft rarely miss 90 but will transpose Runway 7R and 70 instead of 80. When I saw the G5 approaching 90 I issued 80; the MVA. I observed the aircraft descending slowly through 79. I thought he just was overshooting so I told him to maintain 80 and as a precaution issued altimeter low phraseology with the current altimeter. He acknowledged the altimeter and altitude but kept descending. I asked his altitude; he stated 80. I told him I showed 71 and again gave the low altimeter. I told him to climb 1;000 FT. I was not sure what his real altitude was but I was afraid he was low. After climbing to indicate 80 he acknowledged level at 80 and landed without incident. Recommendation; I reviewed the data on my own to see what happened. It appears that even though the G5 had the current ATIS he was 1;000 FT off when he checked in and I missed it. He didn't level off at 90 so I didn't get the chance to see him bust that altitude. I caught the 80 but couldn't get him to stop his descent. We have instituted proficiency days at our facility once a month or so. I believe that has not been enough to remain proficient on moderate traffic. Is there anyway to get a plan to get at least 1-2 hours of moderate traffic without a trainee for a few weeks; I don't want to not train at all because I have some trainees that are at critical phases.