corporate IT administrators

It should still be possible to implement cleartext-with-integrity-protection without changing TLS per se, though you would need an out-of-band protocol for communication between the client and the proxy. The client just needs to give the proxy the decryption key which it derives from the DH master secret. If it keeps the corresponding HMAC secret to itself then the proxy can decrypt the traffic, but not tamper with it; end-to-end authenticity would thus be preserved.