Now Get-WinEvent -FilterHashtable support only Select for query.
Although Windows Event Log supports a subset of XPath 1.0 including Suppress.
An example of such a query:

I see three ways to add new functionality:

1. Add new parameter FilterHashtableSuppress or SuppressFilterHashtable.
2. Use prefix for standard keys: SUPId, SUPLevel, SUPUserID, SUPStartTime, SUPEndTime, SUPData, SUPNamedKey. Ex.:
   Get-WinEvent -FilterHashtable @{Logname="Application";SUPId=1001}

3.Use special 'SuppressHashFilter' property in hash table.
Ex.:
Get-WinEvent -FilterHashtable @{Logname="Application";Id=1001;SuppressHashFilter=@{Id=1002}}