Unfortunately there is nothing yet built in to do what you want. If you know you want to avoid firing alerts at certain times then think your best bet would be to write either a remote java client or CLI script, and execute it on a schedule, like cron, or on demand. Find the resources you want to disable and disable them. Or, alternatively, set the relevant alert definitions disabled.
I expect two things with scheduled downtime.
1) Avoid firing alerts of devices on scheduled downtime.
2) Availability report (I know there ir no report on RHQ). A unavailability in a scheduled downtime is a know and predefined unavailability.
Thank you both for your response. Both response are useful.