Why do you want to do that? If the intention is to input the numbers at the list item levels then that should be the dimension of the module and not the list formatted line item so that Anaplan takes advantage of selective access. And If you have to create a list formatted line item then does it make any difference? The data is stored on some dimension and youa re just adding an attribute - I can be wrong here. If you can share why the need to do this exercise that would be great - I don't think if we can use the selective access in List formatted line items -- there is concept of dependent dropdown that too will not be useful here.
It is a fair requirement to group the read / write list items either sorted or filtered. The customer only want to see the read only items for reference and may want to see only the write items when they make a decision. Having them together is sometime inconvenient.