Today I found this neat way to do it anyway, in the method \Data Dictionary\Tables\LedgerJournalTable\Methods\numOfVouchers:
... sql = 'select count(distinct %1) from %2 where %3 = %4 and %5 = %6 and %7 = %8'; sql = strFmt(sql, ReleaseUpdateDB::backendFieldName(tableNum(LedgerJournalTrans), fieldNum(LedgerJournalTrans, Voucher)), ReleaseUpdateDB::backendTableName(tableNum(LedgerJournalTrans)), ReleaseUpdateDB::backendFieldName(tableNum(LedgerJournalTrans), fieldNum(LedgerJournalTrans, JournalNum)), sqlSystem.sqlLiteral(this.JournalNum), ReleaseUpdateDB::backendFieldName(tableNum(LedgerJournalTrans), fieldNum(LedgerJournalTrans, DataAreaId)), sqlSystem.sqlLiteral(ledgerJournalTrans.DataAreaId), ReleaseUpdateDB::backendFieldName(tableNum(LedgerJournalTrans), fieldNum(LedgerJournalTrans, Partition)), getcurrentpartitionrecid()); ...