You should develop under the assumption that the HttpRequest InputStream is a one-time, forward only stream.
So if you want to read the stream, you need to tell ServiceStack not to which you can do with IRequiresRequestStream or registering a custom request binder, both described at:
Although if you just want to do this for logging purposes you should see if the built-in IRequestLogger is useful:
It is enabled in the SocialBootstrapApi project at:
Which by default is view-able at /api/requestlogs for any users in the Admin role (both configurable).
In SocialBootstrapApi anytime a UserName/Email is registered that exists in AdminUserNames Web.config:
It is automatically assigned to the Admin role:
Cheers,