Here are two ideas for you:
Write a script in for example python using eventstreams
For a downtime end event send a check now api request.
Disadvantages: time periods if used are most likely not checked for that
Alternative:
Use a notification rule for downtime end only
This executes a script that sends the check now api request. Real notifications should have a small delay so the check now triggers before any other notification that might be outdated by that time. Time periods can be used in a similar way as in the check execution.
Disadvantage: use notifications in a way they are not intended