Well, there are two items of interest that apply:

1) The Federal Acquisition Regulation does tell contractors up front about circumstances of applicability in clause 222.225-7040, Contractor Personnel U.S. Armed Forces, etc. This is spelled out in the contract, so there shouldn't be much of a surprise.

2) CENTCOM's lawyers also have the deployed contracting officers putting a notification in contracts warning contractors about the applicability of MEJA.

(NOTE: For legal advice in your state, please consult a lawyer. Sandbag is NOT a lawyer, but knows a thing or two about contract law).