yes, the problem with const patching is not its complexity but the amount of changes needed. i forget the exact numbers now, but a recent kernel has say 200 ops structure types (i.e., candidate for constification), and many more actual variables of said types.
the first problem you'll hit with manual patching is that it takes a lot of time to check every variable instance and their use to determine whether the given type can be made read-only or not (or you'll have to settle on individual variables only), the plugin approach is already a godsend for that reason (in the past another route was tried with coccinelle but that didn't work out too well).
the second problem is patching a given tree *and* forward porting the changes to a newer kernel, including redoing the analysis in the first step since new uses of a given type can come and go any time, meaning that the constifiable property of a type can change either way over time. the plugin approach is again the most efficient way of tracking these kind of changes.
last but not least, once you have such a plugin that can determine when to do constification, it's a very natural step to actually do it from the same plugin (it's like one extra line of code to set TREE_READONLY on the type) at which point one's enthusiasm to maintain a huge patch quickly evaporates ;).
now this was the producer side, but the consumer side is not any better unfortunately. if you read those lkml threads when constification patches were submitted, you'll realize that different developers expected them in different (and conflicting) ways (broken up by subsystem or maintainer or directory or structure type, etc), putting a huge burden on the producer side as if creating the patches wasn't consuming enough time already. then there's an issue with enforcing policy as well, seemingly noone takes constification or checkpatch.pl seriously enough as i keep seeing patches go in all the time that simply don't bother to constify structures.
all in all, while patching the source code is surely a noble goal, my and other people's lives are too short for it...