Hi!
This is a question we often receive. The good news: it is on our backlog and OTK will support JWT access_token in the future. For the current version (OTK-4.1) it is not supported out of the box and has to be implemented by yourself.
To get started you could try a change OTK to issue JWT based token but not validate them since you are not planning on doing that anyways.
Open /auth/oauth/v2/token and look for the assertion "Return Template Response to Requestor" assertion around line 114. It is the one sending the response back to the client.
One line earlier, add an "All assertions ... " assertion and implement your logic to create a JWT. Now, use a regular expression assertion to replace "access_token":"somevalue", with "access_token":"your-jwt", do that on the variable "clientResponse". Play around with this and you will issue JWT based access_token. This is certainly not a perfect or maintainable approach on the long run but it should enable you to experience the behaviour.
Once OTK supports this out of the box you will be able to get this going easily.