Hi Eero!
I just answered on SO with a solution to your problem.
The solution is generic enough to work with all kind of other situations, which should be great for you.
I would like to include it in Genson but don't have very much time right now and want to involve more the library users to become contributors.
If you are interested in contributing it would be great if you could take the time to think about my solution and open a PR so we make it part of Genson.
This would imply making sure it works well with the other features (useClassMetadataWithStaticType to false, runtime types etc), is configurable enough (disabled by default and enabled via a new option in the builder) and is disabled for primitive types (we don't need type info for them, this is already part of JSON format).
If you need help for this I can provide some.
Otherwise I will probably do it later on my self.
Thanks!
Eugen