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.
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.