The General journal uses the similar implementation, where in the lines when the Account Type is selected as customer, the Account will show the customer lookup and for vendor it shows the vendor lookup etc.
look at
\Forms\LedgerJournalTransDaily\Data Sources\LedgerJournalTrans\Fields\AccountNum\Methods\lookup
\Classes\LedgerJournalEngine\accountNumLookup
(in your implementation, you can ignore the company related logic in the above code areas, as this is to support the inter company)