I remember this issue coming up on the Outcome Listserve and someone had recommended the following 2 articles, Anthony, W.A. & Jansen, M.A. (1984). Predicting the vocational capacity of the chronically mentally ill: Research and Implications. American Psychologist, 39, 537-544.
Massel, H.K., Liberman, R.P., Mintz J., Jacobs, H.E., Rush, T. et al. (1990). Evaluating the capacity to work of the mentally ill. Psychiatry, 53, 31-43.
If the mentally ill aren't your primary population, then these refs are off base and you need resources coming from the industrial/organizational psych area.