gryphon, the situation you're in sucks, but it's a business problem, not a technical one. The "let's just fake it" approach is really tempting, but it could expose your company to legal problems. The customer might be unhappy with your technology choices now, but I would expect them to be furious if they were to discover that you were intentionally deceiving them. Check with your legal people before you deploy a configuration that lies about what the underlying platform is (even if your customer has been caught pulling the same trick before).

When letting the contract, if your customer didn't specify constraints on technology (i.e., they didn't say "you must use ours"), then coming back to your company with an additional constraint at a late date should trigger a business-level renegotiation. They're asking you to build something else, after all.

Don't risk getting your company, and yourself, in a lot of hot water.