The short answer to this question is: Yes. However, it is very uncommon. Usually, a 100% disability rating translates to a psychiatric disability so severe as to preclude a claimant from engaging in gainful employment.
Recall that there are two ways to achieve a 100% disability award: by establishing that the veteran’s symptoms fit the 100% scheduler criteria or by showing that his/her symptoms preclude substantial gainful employment. The latter is called a total disability rating based upon individual unemployability or TDIU. The 100% scheduler criteria for a mental disorder is:
Total occupational and social impairment, due to such symptoms as: gross impairment in thought processes or communication persistent delusions or hallucinations, grossly inappropriate behavior, persistent danger of hurting self or others intermittent, inability to perform activities of daily living (including maintenance of minimal personal hygiene), disorientation to time or place memory loss for names of close relatives, own occupation, or own name.
As one can see, symptoms of psychosis or reality problems are needed for a 100% disability rating. Thus, although it is theoretically possible for a claimant to be awarded 100% psychiatric disability and still be working, it is highly unlikely unless the employer is willing to make accomodations. And, of course, if the claimant is engaged in substantial gainful employment (i.e., earning above the poverty threshold), he or she is not eligible for 100% TDIU rating.