I think this would make sense, it would mean you only assign the containing
element on valid elements.

I was thinking something along the lines of:

#define list_for_each_entry(pos, head, member)					\
for (struct list_head *list = head->next, typeof(pos) pos;	\
     list == head ? 0 : (( pos = list_entry(pos, list, member), 1));	\
     list = list->next)

Although the initialization block of the for loop is not valid C, I'm
not sure there is any way to declare two variables of a different type
in the initialization part of the loop.

It can be done using a *nested loop*, like this:

#define list_for_each_entry(pos, head, member)					\
	for (struct list_head *list = head->next, cond = (struct list_head *)-1; cond == (struct list_head *)-1; cond = NULL) \
	  for (typeof(pos) pos;	\
	     list == head ? 0 : (( pos = list_entry(pos, list, member), 1));	\
	     list = list->next)

I believe all this does is get rid of the &pos->member == (head) check
to terminate the list.

Indeed, although the original way is harmless.

It alone will not fix any of the other issues that using the iterator
variable after the loop currently has.

Yes, but I stick with the list_for_each_entry_inside(pos, type, head, member)
way to make the iterator invisiable outside the loop (before and after the loop).
It is maintainable longer-term than "type(pos) pos" one and perfect.
see my explain:
https://lore.kernel.org/lkml/20220302093106.8402-1-xiam0nd.tong@gmail.com/ (local)
and list_for_each_entry_inside(pos, type, head, member) patch here:
https://lore.kernel.org/lkml/20220301075839.4156-3-xiam0nd.tong@gmail.com/ (local)

--
Xiaomeng Tong