Pagination - Text comparison with greater than and less than with DESC
I don't think just placing an arbitrary value would work, since you have to put a larger if not possibly the largest value.

Perhaps pass in a boolean value with an OR operand (WHERE someflag OR last_name &lt; @key), than way you do not have to pick arbitrary lowest value and largest values.

Otherwise if you can only control what is passed and not the WHERE operation, you'll have to use whatever the max value is for the column based on encoding.