Hello,
I have checked in my system and I see we do not have the order unit field available to create condition table with it but then we have a option to enahance the field catalog and there if we mention the field . However even in feild catalog I cant locate this filed but I see we have filed dummy.
Conclusion: First step will be to do enhance field catalog with order unit field. If that is not possible then we need to use dummy field in field catalog to create condition table and then we need to pass order unit from MB1B screen to batch search/selection procedure.
I know you have already tried for exit and all but I do not see any way possible to pass order unit to your selection /search procedure.