Access management for DB project.

Hi,

I started thinking of some security access management. Basically imagine
this scenario according users:

1) Writer does only inserts to black hole.

2) Reader does only reports on inserted data, cannot modify or add
anything

3) Maintainer can run a task on the data, but cannot read or add anything.
The task has to have read/write access to the tables.

The first 2 types are easily solvable, but with the third type I have
problem. I have created task in plpgsql, I granted permissions to an user
to execute the task, but revoked on him all rights to tables. Logically
task failed.

The task sits in different schema, but operates on tables in other schema.

How would you solve this task?

Regards,
Bohdan