A lot of progress has been made. I've adapted gui_selectkeys widget into a full menu with buttons to set a global selection hotkey for any factory-made unit.

The simplified version is almost fully realized except that hotkeys can't be shared across multiple buttons for unit selection. If you're reading this and you have a solution for selecting every unit that has a hotkey assigned to it, let me know!
The workaround is, in addition to the global selection menu, to create custom unit group lists with a single key that represents a unit role. This is sometimes very straightfoward and other times a judgement call as to which unit should belong in which category. The good things is that a lot of different groups can be made to suite a diversity of preference.
The intention is to be using spacebar as the modifier key for unit selection since it's unused by default.
I'll be excluding Air and Naval units as these should get their own specific hotkey groups (working on it).
The first idea I had is to simply use the grid layout from factory production.


And a lot of that works without a hitch for Constructor (Q), Skirmisher (E), Riot (R), Assault (T), Artillery (Y), and Anti-Air (D). This is great because players can develop a muscle memory to select these roles globally regardless of what factory they plop first, and it frees up your valuable Control Groups and Auto Groups to be used situationally while overcoming the biggest barrier to hot key useage: memorization.
But some unit roles are messier and sometimes the unit role doesn't cleanly fit.
Scouts (S), Raiders (W), and Heavy Raiders (using (A) since it's free on the grid). While I'm tempted to throw everything that sits on W in the factory grid into a single grouping, I think it's actually more practical to split it up in this case. Here's what I've come up with, anything in
italics has been moved off of it's factory production grid letter.
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Raiders (W): glaive, bandit, duck, scorcher, pyro, *bolas, *venom, *kodachi
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Scouts (S): dirtbag, puppy, dart, *flea, *dagger,
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Heavy Raider (A): *Blitz, *Archer
I understand that Flea and Dagger were put into their first spot in the factory to communicate their greater viability to players. Flipping the positions in the grid of flea/venom and dagger/bolas would correct this inconsistency in the grid layout.
I'm sure there will diversity of opinion on exactly what the category groupings should be.
That just leaves a few categories:
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Signature Units (F): phantom, crab, felon, jugglenaut, impaler, cyclops, grizzly
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Bombs (G): Imp, snitch, skuttle, limpet
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Utility (H): Iris, aspis, djinn
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Weirdos: Widow, scythe, dominatrix, tremor, lobster, claymore, striders
I think these lettered groupings are all useful, even if less useful than the prior groups. Players are not likely to often have multiple signature units from different factories in a game so it's helpful to have a button that will select your Grizzlies when playing Amphbot and your Phantoms when playing Cloakbot.
The weirdos are the units that are more suited to having specific hotkeys or being put into Auto Groups.
Once all these special groups have been added I'll be posting the widget for people to try out. Let me know if there's a specific grouping you think would be good and I'll try to prioritize the ones that have the most popularity. All feedback or discussion appreciated.
Now that I've got the basic idea working and I'm testing it, it feels GREAT to be hot selecting my anti-air with Space+D or retreating my Recluse Ball with Space+E without having to think about it.