NFPA 79 specifically addresses e-stops. Use this as your guide. E-stops must be accessible and NOT covered or recessed. OSHA does not directly spell this out, however, under their Articles of Incorporation they can use the NFPA 79 Standard on E-Stops to cite companies.
If you do not have NFPA79, a very good discussion on E-Stops is available from Square-D.
Please go to the following Square-D link to open and review pertinent standards/requirements presented in Appendix C pages 319-322 of their Safety Catalog: