Is there a straightforward way to get a list of direct reports for a supervisor?

Are you using additional Groups to define users who do particular task?
You can, in that case, only look for users within those groups to identify if the supervisor is the one who is logged-in and return the value.
You can do that in a single expression using the applyFunction to loop over the result set.
Ideally, you should be using Group based assignment along with using the correct process instance security that would allow the reassignment privileges to a user with Manager credentials.

Are you using additional Groups to define users who do particular task?
You can, in that case, only look for users within those groups to identify if the supervisor is the one who is logged-in and return the value.
You can do that in a single expression using the applyFunction to loop over the result set.
Ideally, you should be using Group based assignment along with using the correct process instance security that would allow the reassignment privileges to a user with Manager credentials.