Hi Jill,
I suspect the reason that the Percent Complete and Assignment are read only is that they both "live" at the Task level, one level up from your Assignment report. Instead, if you wrote a Task level report, both the Assignments column and the Percent Complete would then be inline editable.
If need be, you could then also add an iterate column to pull anything of interest "up" to the row (read only, mind you) from any underlying Assignment(s).
Regards,
Doug