Sorry for late reply.
This issue had not been solve
This might be possible to do but not on a collection object.
So in our system we have a number of datetime fields for users to populate i.e. desired start date, desired end date. They first populate this information as part of a process collection action.
Using an automatic windowed windowless action (I dont have a better way to describe this ) I copy the value back on the main form, you can't use runtime values so have to use calculations to populate the field.
Because the calculation on the field isn't at the object designer level it leaves the field editable by the user.
Hopefully this helps to point you in a direction that gets it working for you.