Thank you, Steve.
Okay, so I've been thinking about this too and I kind of came round to a PC gaming metaphor. In games I used to play like Carmageddon, there was a screen where you could customize key combinations for certain actions. For instance, SPACE for accelerate might be the default but you could change it to ALT or whatever.
What I'd like to see in browsers is a browser chrome menu option which is already keyboard accessible where you can easily set custom keyboard commands for certain actions. So maybe there'd be a field for landmarks where you could focus an input and simply press a combination of keys and those keys would be set. I think I might set ALT + TAB for going to the next landmark and ALT + SHIFT + TAB for going to the previous landmark. The UA would then apply a UA style for focusing the landmarks via these shortcuts.
You could have an option for saving kbd shortcut combinations just for that web page / domain or as default for all your browsing and could specify overrides where you felt certain pages needed different shortcuts.