How to use variable value for Rows Per Iteration? (1.7)

I'd like to be able to use a variable value for the Rows Per Iteration field that was introduced in 1.7.0. I've been able to do this for other fields in the UI, but when I pass in a property to this field, it resolves to the actual value and leaves it that way. In other words, I've got a property ${DataSource#count} that I put in that field, but when I exit the dialog and return to it, it's been set to the actual value (e.g., "1"). 

Thanks!