To try to logic this one out - adding a slip on buttpad wouldn't permanently alter the length... or would it... I would think not since it could easily be undone without tools.
That'd be a no - as you say not permanent. And if you had a 26 inch rifle, took the buttpad off and then put the slip on in it's place even tho the length was now the same it would be prohib'd
Adding a new full length butt stock.... it isn't altering the current wood on the gun, and does make it over 26 inch OAL... but can't be undone without tools... even if the tool is just a screw driver and a screw.
Replacing a part with another factory part isn't considered to be alteration. I asked the cfc about that once. You are not 'altering' a gun by putting a factory part on it, especially if it returns it to it's original factory form.
Otherwise, just replacing a broken stock would constitute 'alteration'.
Adding an attachment that could be added and removed without tools and would leave the gun at the exact same OAL when not attached, but over 26 inches when attached..... how do you deal with that one??? IE A slip fit MASSIVE butt pad
It's removable - it's not altering the gun. No more than simply duct taping a stick that pointed forward of the barrel would be 'altering'

(we could get around all kind of length restrictions doing that

)
You start to get a little 'grey' when you get into things that actually attach, but even there the cfc has always had a policy that it has to be 'permanently' attached. For example - they will consider a muzzle break as part of the length of a barrel, as long as it's welded on or otherwise 'permanently' attached to the barrel. But - they are fickle in their ways
Now - the law does mention that it cannot be reduced by 'telescoping or folding' - so you wouldn't want anything that could do that.
But imagine you made a 'clip' on the short stock of the ranch hand - and were able to attach a metal rod that came back to another base plate for the purposes of quickly turning it into a full length carbine. It would just pop off, so is it an 'alteration changing the length of the gun? I think it's safe to say no.
What if you put a threaded hole into the rather useless butt plate of the ranch hand so that you could screw such a device on and off? Well.... still seems like no. It doesn't telescope or fold or the like.
Put a device that folded out of the way tho... now it's a prohib.
I suppose if a guy was so inclined it wouldn't be hard to drill a screw in hole on the back of the stock such that you could attach a short 'extension' that would come all the way back to your shoulder with a proper recoil pad mounted, and thus turn the gun from a micro-rifle into a proper carbine when necessary. It just wouldn't be 'instant' like a folder.