This application talks to reservation systems, some of them accept utf8 but some others -really old- only accept ASCII.

I think that, rather than considering how to turn UTF8 into ASCII, the real problem you're facing is how to send your data to those "really old" systems in ways that they would find acceptable and readable. Some years ago, I did some work for a medical data processing company that had a very similar problem/question that they were trying to solve. The "solution" they had been trying to implement was a single "unified" format for everything (they'd spent a couple of years on this already, lost a number of customers, and weren't much further along than where they started.) My answer, though it sounded inefficient, was to simply forward all the "acceptably-formatted" data to the companies that could use it, and write custom converters for each of the rest. We were done in just under two months, and handled every single format (yes, it was done in Perl. :)

In short, I suggest that you send the UTF8 data to the companies that are happy with it, then contact the ones that only accept ASCII and get the precise definition of how they want those issues handled (obviously, they have some way of doing so - and more importantly, have already decided how those issues are to be handled in their case, meaning that you don't have to reinvent that wheel.) Write the necessary converters for those cases, and send that data to those companies. Done deal.