Surely certification is now done by EASA, and if something has been approved by another EASA member (even at any time before EASA came into being) then you should have automatic approval.
The obvious difficulty with the process is locating aircraft owners who have had something fitted previously. There is no central database for Europe. A starting point for research would be contacting the manufacturer of the said equipment and see if they will disclose contact details of somebody in the EU who bought the product - this is likely to fail if it was a non-cooperating maintenance firm or some other dead end.
The CAA have a database of AANs (on their website) but it's obvious, looking at a few cases which I happen to know about, that a lot of stuff got fitted to G planes over the years without anybody obtaining an AAN - largely because "everybody" thought that if something has a US STC it could be screwed to a G without formality. This was true under the now-defunct CAA-FAA treaty but you were still supposed to get an AAN, and the CAA could have refused (and often did) on major mods.
In your case, you will need to find somebody on F who has the skis fitted and get the DGAC reference from him.
I hope the above is correct; I don't work in the business and now I am on N it doesn't affect me anyway.