Using Lookup And Display Only Field As Control Field

I am not sure If I can do this with a lookup, but what I would like to
do is perhaps use lookup to retrieve a control date from an
unassociated table to control what date is entered in another table.
For example :
the main table , table 1 has many entries with a field called date_
enter which is the date the record was entered.
table 2 has a control_date
If the date entered in table 1 is less than or = to the control date we

want to give the user a error message.
I am thinking of using display only field and lookup to set it
Would need to have the date value from the control table available to
the active table of table 1 when entering the the date_enter.
However as there is no join field between the two tables am not sure
how to do it. Was thinking might have to add a key field that was
allways null and in the BEFORE EDITADD EDITUPDATE section set it so
that the key would be null

Am using Informix 5 , Any help would be apprecia