It should be possible, and I've been planning to do it for a while. I need a lvl7 modifier for my second Extend layer. But I haven't gotten around to it yet.

Recall how I finally managed to remap Ctrl+Z and Shift+Ctrl+Z for Extend:

Redirect(key=<AB01>,mods=Control,clearmods=LevelFive), Redirect(key=<AB01>,mods=Shift+Control,clearmods=LevelFive), // Ctrl+Z

That was the craziest bit of XKB moddery I've performed so far, for sure! ^_^

So I think this action should work:

SetMods(mods=LevelThree+LevelFive)

But I haven't tested it yet. Let me know if you find out something!

Hmmm... check out symbols/level5("lock") and types/level5("EIGHT_LEVEL_LEVEL_FIVE_LOCK"). There it says that you need a "real" modifier key. So I guess we could do what they do, more or less, which is to cannibalize for instance the NumLock modifier. I'm really not familiar with the "types" definitions though. It may be that their need for the NumLock modifier was mostly driven by the desire to use its LED to show the lock state...?

// This adds the definitions needed to create a level5-lock behaviour, using
// the real modifier NumLock as a lock indicator.
// See also: types/level5 : EIGHT_LEVEL_LEVEL_FIVE_LOCK
// See also: compat/level5(level5_lock)
partial modifier_keys
xkb_symbols "lock" {
  key.type[Group1] = "ONE_LEVEL";
  include "level5(modifier_mapping)"
  replace key <HYPR> {
    vmods = NumLock,
    symbols[Group1] = [ NoSymbol ],
    actions[Group1] = [ SetMods(modifiers=NumLock) ]
  };
  modifier_map Mod2 { <HYPR> };
};

The level5 state is Mod3.

Note in the above how they didn't map the <HYPR> key to a key symbol but directly to the SetMods() action. That's how I did my Extend mappings too.

Maybe if I were more clever with my types definitions, I wouldn't have to add all those clearmods=LevelFive entries...

The big question for me is: If I setmods differently for, say, the Caps key's first and third levels (AltGr+Caps), will the Caps key retain the set mods until released even if AltGr is released? Because that's my preferred way I think: AltGr+Caps selects Extend2, Caps alone holds it.

Maybe all that majik with the "real mod" isn't necessary for us, or maybe we'll eventually discover that we've fubar'd something by our simple ways. Time will show.

Mouse keys are a bit sucky. I think there are some accelerated and diagonal mouse keys that might help though. In XKB, it's simple to create your own using the action syntax in my extend file:

MovePtr(x=+0,y=-8)

Maybe a decent mouse layer with diagonals and a combo of big and small jumps could be nice?

Instead of UNEI, I'd be tempted then to use E as mouse button 1 and H/,/. as the downward movements. Maybe M and O as middle/right button?

Maybe like this?

| J    | L    | U    | Y    |
| K    | N    | E    | I    | O    |
| M    | H    | ,    | .    |
| Spc  |

| WhUp | LeUp |  Up  | RiUp |
| WhDn |  Le  |  Dn  |  Ri  |  M3  |
|  M1? | LeDn |  M2  | RiDn |
|  M1  |

Acceleration on Shift sounds nice to me!

Shift-click M1 on M? I think M1 on Spc is good, as it's very easy to hit with the thumbs. But I'd also keep the left-hand home row mods for the mousing layer.

A separate Extend2 key would be nice, but I don't feel I have one to spare... And as we approach Extend3 and maybe Extend4, the problem compounds. I use Extend2 for NumPad, so my mousing layer if I make one would have to be one of the latter.