i have been thinking along the same lines and i agree with most of what was suggested. i personally have found a (incomplete) solution for myself but it is very personalised and needs a lot of time and effort to do initial set-up. but... but for me it is without any hesitation the single most useful workflow enhancement though unfortunately implemented by me and needs a relatively expensive third party software.
i was talking about this in the following topic:
archicad-talk.graphisoft.com/viewtopic.php?p=205385&highlight=#205385
"after customizing all the most used commands, i virtually run out of normal keyboard shortcuts and it's not possible to memorise if cmd-ctrl-shift-S is assigned to "set home zoom" or "send backward" or what have you. so i was looking for system that can simulate autocad style commands where you have to type in first few letters of the command to distinguish it from the rest of the commands beginning with the same letters, so one wouldn't have to remember all the tedious keyboard shortcuts possible. i found few workarounds to achieve this on mac. programs like quickeys, keyboard maestro etc came to help where you can use abbreviations to type in shortcuts, press menu items or buttons etc. so now for example typing in "irl"(invert reference line) executes design menu>modify wall>reference line, clicks invert all sides and simulates pressing enter to close the wall reference line window, without having to use the mouse at all. of course it took some time to set this all up in quickeys, but without a need to use the mouse for going through endless menus and windows and buttons, i think it payed off in a week or so and has really improved my speed in archicad. it's much more simple to memorise that typing "sf" means sun off and "sn" turns sun on in 3d or that i can even change layer combination to "render" or "work" layer combination by just typing in "re" or "wr"! or "em" to edit hotlink modules or "um" to update selected module etc etc. all of this usually means opening a number of windows from the menus and making a numerous clicks. i wish this could be an integrated option in archicad to be less mouse dependant..."
so in my case manipulating behaviour with the key sequences(HL-hide layer) on a single layer level is limited to the quick layers palette, meaning you can turn off the layer of the selected element and turn it back on only by using undo quicklayer action button(UL-undo layer). but on the other hand which works extremely efficiently is layer combination by assigning key sequence to them and this due to the fact they are found in the menus, that quickeys can reach.
quickeys is also available for windows, but i cannot comment on it's functionality compared to on a mac, but i guess there are more alternatives on windows platform like autohotkey etc etc. it's just that quickeys seemed to be with the most functionality and most easy to set up.