Right now the cybernetic combinator set to wagon info sets the filtered slots, but it doesn't lock the non-filtered slots. Meaning you can insert above the limit, and can also insert wrong items into the other slots.

The issue is of course that if your inserters are holding items when the train arrives and they can't put it in you're deadlocked. So I'm not sure what is preferred here.

I think the documentation tries to lay out the design vision for this mod, in this case, by suggesting the use of a picker to remove overloaded items. This is in the part of the docs that mentions use of a "memory cell".

This is intentional behavior. It's inconvenient I know but it's the only sane way to make universal multi-item stations function. The intended way to implement multi-item stations is by using inserters that are set up to never let inserters hold items after a train leaves. You want to use inserter circuit conditions as opposed to filtered slots to control the amount of cargo that gets loaded.
Connect the inserters to the output of the station combinator, and subtract from that circuit network the contents of the parked train. Set the inserters to only load items when that item has a positive signal. The non-filtered slots aren't locked specifically so those inserters can overload the train by a small amount so they don't get stuck holding items.