Julian,
No personal experience with this, but reading the OGN forum (
https://groups.google.com/d/msgid/openglidernetwork/af4b77ee-a5af-44d0-96ea-17a4766bec24%40googlegroups.com)
It states that ICAO ID’s are not used by FR24, to quote from Sebastien’s post
“FR24 doesn't use Flarmnet database.
They use data coming from ddb.glidernet.org. (but i'm not sure about when they retrieve the content)
If you use ICAO ids, it will not be visible on FR24 as they filter this type of id.
We send all data to FR24, but they apply some filtering. Tou need to check with them to know more.”
Hope this helps, it is probably better to post the query on the OGN forum, then one of the developers can give you a definitive answer.
Ben.