I think (remhash 1114 cl-postgres:*sql-readtable*) is what you want -- where 1114 is the OID of the timestamp type. Cl-postgres registers a binary interpreter for that type by default, since that makes it easier to stuff these values into custom datatypes (see set-sql-datetime-readers). Unregistering that interpreter-function will make it retrieve the values as strings.
Best, Marijn