Some parts are big enough to require ten, fifty or more kerbals to move around. I can position them with a tug or robot arm, but no matter how much mechanical help I have handy, I still need the same number of kerbals to actually connect the part up.

One simple way out of this mess would be this: If a grappling node (stock or KAS) is as close to the part in question as a kerbal would have to be, we assume mechanical help is available and the number of kerbals necessary to move the part is reduced to 1 for every 10 tons of weight. This is of course a bit cheaty, but the logic could be improved upon, e.g. depending of the mass /TWR of the vessel that has the grappling node, which would in turn raise questions on how you calculate the TWR of a mobile crane which is essentially a rover and has no real TWR... let's just introduce tugs as a general concept into KAS and sort out the fine print later, eh?

One thing that could really be an issue: I have no idea how the game / KAS reacts if a part that is already attached via a grappler (and effectively already part of the vessel gets attached again via a node. If that becomes a problem, it could be solved

• by detaching the grappling node at the same time the part is attached to the vessel
• by stipulating that one can not attach a part to vessels that are already grappling it (but that would break manipulator arms to no end)
• by stipulating that the grappler may only be near the part but not actually connected to it to count (which would be kinda stupid)

It looks more like a KIS feature. KAS doesn't attach arbitrary parts together. Building large structures using a kind of crane is an interesting idea. Though, in the current state of the mods it's unlikely feasible. There are to many things to be designed to turn it into a feature instead of another cheat that ruins gameplay.

Btw, if you're OK with "kind of cheat approach" just increase mass/distance limits in KIS.