I think the BASIS-32 can be modified and works well for patients in a day treatment program as well as for inpatients. However what used to be called the American Assoc of Partial Hospitals recommends several supplemental instruments for geriatric patients. To assess functioning, you could examine the Katz Activities of Daily Living (Katz, Ford, Moskowitz, Jackson & Jaffe (1963) Journal of the American Medical Association pp. 914-9. or the Lawton/Brody Instrument of ADLS Lawton & Brody (1969). Gerontologist. 179-186. To assess symptoms, the Geriatric Depression Scale (Yesavage, Brink, Rose, Lum, Huang, Adey, & Leirer (1983) Journal of Psychiatric Research pp. 37-49) might be an option. There are lots of different measures, but the advantages of these would be that a national association endorsed them. Still, if you are looking for a good clinical measure for diagnostic purposes, these might not be helpful. Perhaps someone who does a lot of psychological testing might have some suggestions.
Are you doing any routine assessment now?