Having done the equivalent for a gnarly enterprise web-app when IE8 came out (using mod_headers to inject the HTTP header equivalent) this isn't trivial and they may not have anyone with the necessary skills who is jumping to take on testing and support (
everything which happens with that app for the next 6 months will be first blamed as an upgrade problem).
Beyond that, it doesn't actually work: some features won't be completely compatible so you're effectively creating a third browser to test and support. In the case of IE8, it was mostly IE7-compatible but it relied on the browser not throwing errors when JavaScript attempted to set syntactically invalid styles and IE8-in-IE7 still raised exceptions deep in a third-party library licensed by the app vendor. This app was made by a very large, wealthy company – we were paying mid-six-figures a year for "support" – and they preferred infrequent massive update releases, which meant that a week after I reported the bug (around the time Microsoft started pushing IE8 as an automatic install) one of their managers contacted us asking for the patch I wrote so they could redistribute it to other customers.
This is why I won't work on enterprise systems.