You can give design time and runtime permission to set of user roles , additionally there is a provision for service overides for particular User.
I think you have requirement to show the html pieces for set of user roles , you can make custom services add in mashup , use expression widget
to visible True or False .
var result = Resources["CurrentSessionInfo"].GetCurrentUserOrganizations();
var result = Resources["CurrentSessionInfo"].GetCurrentUserGroups();
Some of snippets , you can look for Resources methods for Currently logged user .
In addition, please refer to the following video on more information regarding permissions and "deny all" default policy, https://www.youtube.com/watch?v=HzFqxvgHtpI&index=8&list=PLz1ppcU_kaneagUT9qgQfz3HByf6-9zTF
You may also look at this document on the use of system user The use of System User