For the second case, you can use

a, *L[::2] = "abc"

For the first case this does not work, because an assignment can have only one starred expression.

Making the first case to work as you expected is breaking change. Currently

L[:], *rest = 'abcdef'

sets L to ['a'] and rest to ['b', 'c', 'd', 'e', 'f']. Consistent implementing of your idea would set L to ['a', 'b', 'c'] and rest to ['d', 'e', 'f'] (because len(L[:]) == 3 before assignment).

What is your use case? Why do you need such syntax?