When you want to construct a query-object that contains OR-conditions, you can construct something like this:

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queryBuildRange.value(strFmt('((%1 == %2) || ((%1 == %3) && (%4 == "%5")))',
                    fieldStr(InventTable, ItemType),
                    any2int(ItemType::Service),
                    any2int(ItemType::Item),
                    fieldStr(InventTable, ProjCategoryId),
                    queryValue("Spares")));

(you can find more info on Axaptapedia: Expressions in query ranges)

Now there is a limitation to this way of working. When the generated string in the queryBuildRange.value is longer than 250 characters (the length of the range EDT), Ax cuts the generated string off when you open the ‘modify query’ dialog. I have tried to enlarging the EDT, but is not a good solution. The error does not occure when you open the ‘modify query’ dialog, but it occures when you close it.
A small (but limited) work-around could be to make the name of the datasources on the form/report where you want to apply this trick smaller. I hope to find a better solution, but this does the trick for now.