External endpoint are invoiced differently. If you are looking for a different solution which doesn't affect your invoice. You can still reach into your slots via the traffic manager using the ?x-ms-routing-name=_slotname_
query string. This generates a cookie entry and redirects all consequent requests to the defined slot, until reset.
For example you could have a "join test" button and let your users opt in to your beta app by clicking on it:
<a href="yourtdomain.trafficmanager.net?x-ms-routing-name=test">Join Test</a>
To return to your production slot, you can call the routing name self, which refers to the production slot at all times.
In order for this setup to work, you need to have set traffic routing in your web apps behind the traffic managers. These routing entries can be with an assigned 0%, so that traffic is not automatically redirected to them.