You may have seen this announcement:
The next major v4.0 will also be a "breaking-release", with new features protected under an "AGPL/FOSS License Exception" and include a number of breaking changes that have built up over time, including upgrading all projects (to .NET 4+), major code and NuGet package re-factoring, clean-up and removal of deprecated code, better consistency / simplification of some concepts and implementation of some roadmap features I've been wanting to add but never had time for. Fixes and releases will continue to v3.x and be maintained in a separate v3 branch until v4.0 comes out of beta.
As there's no reason to fix dynamic object support in the current 3.x branch, which doesn't support .Net4.0, the answer is 'No, ServiceStack can't serialize the JSON request into a dynamic.'
You can always fork the code and add the missing support in a ">= .Net4.0 only" branch. A pull request with that fix might not be accepted into the 3.x core, but might be welcome for the coming v4.0 release.
Alternately, there are hooks for custom serialization functions in ServiceStack's JSON Serializers, JsConfig<T>.RawSerializeFn
and JsConfig<T>.RawDeserializeFn
. In combination with this answer on serializing ExpandoObjects in .Net 4.0, that would allow you to add ExpandoObject serialization support without having to 'fix' the core code.