I came across this article on a post elsewhere - I have employed this mechanism to achieve what I need - it is actually very easy to do and makes sense.
It is actually very easy to combine multiple datasources in a sysTableLookup. Here is the trick I used to be able to filter on the name from the EcoResProductTranslation in the lookup for items.
1) Create a view that combines all your datasources and add the fields you would like to see in your lookup to the view.
2) Create a query from the view created in step 1.
3) Use these to perform your lookup as follows...
static client void lookupItemActive(FormStringControl _ctrl)
{
SysTableLookup sysTableLookup = SysTableLookup::newParameters(tablenum(<ViewName>),_ctrl);
Query query = new Query(queryStr(<QueryName>));
sysTableLookup.addLookupfield(fieldnum(<ViewName>, ItemId));
sysTableLookup.addLookupfield(fieldNum(<ViewName>, Name));
sysTableLookup.addLookupfield(fieldNum(<ViewName>, ItemGroupId));
sysTableLookup.addLookupfield(fieldnum(<ViewName>, Status));
sysTableLookup.addLookupfield(fieldnum(<ViewName>, RevId));
sysTableLookup.addLookupfield(fieldnum(<ViewName>, ItemType));
sysTableLookup.parmQuery(query);
sysTableLookup.performFormLookup();
}