PowerShell is your best best:
Get-Cluster -Name "Cluster1" | Get-ClusterResource | Where-Object { $_.ResourceType -eq "SQL Server Availability Group" } | Set-ClusterParameter -Name "LeaseTimeout" -Value 600000
There are ways to enumerate a list of cluster names but I don't know them offhand. In any case you can generate that list and then pipe it to the Get-Cluster cmdlet and it will process them all.
You may have to install PowerShell remoting on the cluster nodes in order for this to work properly, I haven't tested it on more than 2 clusters myself.