Custom Reviewer selection in User Access Review

There are two different things you asked about here:

1. Select reviewers using a custom query.

Yes that is possible using coverage files. What I've seen in many places is using a the custom query to create the coverage file. So for example, assuming that your custom query to get the reviewer vs the user is:

                select REVIEWER_USER_ID, USER_ID from MY_CUSTOM_REVIEWERS_TABLE

                Then you can create another query using this that would output the structure of your coverage CSV file:

                select 'user_id=' || REVIEWER_USER_ID || '|user|user_id=' || USER_ID || '|application|1=1|' as coverage_file from MY_CUSTOM_REVIEWERS_TABLE

                The output of the above query, if exported as a CSV file without headers, can be directly used as a coverage file. You can of course change the structure of the query as you wish.

2. Self-coverage for reviews. This is available as a checkbox in the User Access review definition. On the reviewers tab of the review definition, make sure you check "Allow reviewers to review their own user entitlements” then also the child checkbox "Users review their own entitlements”.