Unfortunately this isn't possible. The SearchFilter classes in the EWS Managed API ultimately have to serialize into SOAP XML that conforms to the Exchange Web Services schema. For example, a SearchFilter.ContainsSubstring serializes into a Contains XML element in the SOAP request. You can see examples of all of the Managed API classes and equivalent SOAP here: http://msdn.microsoft.com/EN-US/library/office/dn579422(v=exchg.150).aspx.
So in order to send a filter via SOAP, it has to conform to the schema, which means it has to be one of the available filter types. There's no way to send a custom type. In order to do custom filter processing, you would need to pull the relevant data down to your client, and do the filtering there.