Yeah, new AP.jira.showJQLEditor() API is of no use in current state 
I see couple of obstacles to implement user friendly lozenges instead of plain accountID in jql by vendors now:
- no regular expression for accountId (Regular expression for the new atlaasian account ID - #2 by david2)
- Let say we are able to extract accountIds from jql → there is no bulk API to get display names for accountId yet ( [ACJIRA-1624] - Ecosystem Jira in progress)
One more thing: it seams, it is not possible to prevent users from using user keys instead of accountIds. Suggestion REST API returns accountIds but it is possible to pass user key by typing and validation passes (rest/api/2/search?jql=assignee+%3D+admin GET 200, even with apiMigration=true). So whole migration of jqls seems to be pointless unless after each edit vendors uses /rest/api/3/jql/pdcleaner, which furthermore has others drawbacks