One final thing. Under no circumstances should you or your boss reveal to your counterparts in Frankfurt that you are flying to/from Hahn. They will likely regard you as unreliable cheapskate muppets and not suitable as business partners. Just let them assume you flew to the main Frankfurt airport like any normal business people instead.
On the basis that, for a business traveller, time is money then it is difficult to envisage how a low cost flight to Hahn for business in Frankfurt could ever make sense.
Yes short notice flight from UK to Frankfurt may be in the £400 to £500 bracket. But if you factor in the cost of four or five hours extra travelling time for the individual before even thinking about the cost of rail and taxiis then I do not see a flight to Hahn ever being cheaper in real terms.
Cheapskating on Ryanair only makes sense for the private flyer who chooses to sacrifice a lot of their own time to save the money.