The problem with cross-chaining is that you put more stress on the chain which causes wear and tear on cassette teeth, eventually leading to the chain slipping gears.
The chain line is one factor. The chain should ideally run in a plane from the front derailleur teeth back to the rear derailleur. It shouldn't be forced into a line on the teeth, and then angle as soon as it clears the teeth to get to the cassette as happens when cross-chaining because that causes teeth to wear. You're not going to fix the chain line except by making the chain and gear spacing narrower which requires weaker chains at a given price.
The length of the rear derailleur cage determines how much slack it can pick up in the chain. If you size your chain big-to-big, and have a derailleur capable of picking up the slack then shifting into big-big shouldn't do damage. Shimano XT rear derailleurs have a 43 tooth capacity which should cover just about anything, but I had a touring bike with a wide range of gears that exhausted it. I managed to wedge things pretty well when my shifters got pushed into awkward positions on a train and I started riding without checking.