if I understand you correctly, you have like two boxes or a straight_table or something where the results from table_B are displayed based on the selection in table_A?
=> Can't you put an expression there using ISNULL() to display "unknown" or anything in case there is no value to be displayed?
And just to post from a different point of view:
One of the best practices in data management is to seek completeness at all times. This means that missing data (cf. unknown product IDs, missing customer groups etc.) will be marked with clear indicative values (like the one you suggest: 'Unknown') at load time. This can be pretty simple and is very helpful for end-users to interprete the presentated data in a correct way.
What to do?
To table B, add all missing IDs that are present in table A, and give them field values like '*MISSING*' or 'Unknown' or 'Unassigned' Example:
LOAD ID_A as ID_B
'Unknown' AS Field1,
'Unknown' AS Field2,
WHERE Not Exists(ID_B, ID_A);
and then do the JOIN.