The answer to this question is found in the Quality Assurance Plan (QAP) for the instrument in question. The QAP for Intoximeter instruments recommends the use of dry gas, but also allows the use of wet bath simulators as well. Simulators must be approved by NHTSA (by appearing on the Conforming Products List of Calibration Units for Breath Alcohol Testers) and by Intoximeters, and simulator solution should be certified and properly maintained. The simulator solution used should produce a 0.040 alcohol level.
Note that QAPs from different manufacturers will differ. Consult the QAP for the breath tester in question to determine if simulators are acceptable for use for DOT purposes.