From an employment law perspective only, it may be a violation of national, state, and/or local workplace safety laws not to provide employees with adequate restroom facilities in the workplace. The federal Occupational Safety and Health Administration (OSHA) regulations provides that, except in limited circumstances, "lavatories shall be made available in all places of employment." See 29 CFR 1910.141 for toilet facilities guidelines in the workplace: http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9790.
Liability for violations of employment laws falls to the employer, which, depending on the circumstances, may sometimes include the individual supervisors and managers. Maybe your employer can simply get a key to that restroom for after hour employee use only.
This is not legal advice. For legal advice, you need to consult an attorney regarding your individual situation.