To better define the root problem.

Currently we have a flow which involves a lot of ping pong between developers and qa engineers.

When developer fixes bug / creates new feature to test, he changes status of a maniphest task to "QA Pending" and assigns ownership to QA department team lead with their own workboard and processes which are out of scope of dev.

The ping pong part comes to play when developers don't provide enough info to QA or the bug is not really fixed, feature is not working as described where QA tester reassigns back to the developer and changes status to OPEN.

To fix that a bit, we had an idea to add a custom Maniphest field which would fix the change of statues and reassignment of task owners where both the dev who is assigned to the bug and tester who's in charge of quality are on the same task accountable for their part. Maniphest allows for this but only supports type: users field which is a multiple users type-ahead where we would only like to have it fixed to a single user like "assigned to" field is.

Do you support it but it's hidden or undocumented somewhere or is there any other workaround you suggest?

"custom_field:tester": {
  "name": "Assigned Tester",
  "type": "users",
  "caption": "Assigned Tester",
  "required": false,
  "search": true
}