I'm an idiot because I didn't think to look at how it adjudicated magic - probably the biggest thing keeping me from implementing HOTT style mass combat into my games using PCs. It wasn't including PCs into the elements that bothered me, no, I handle that like this:
When a PC/NPC becomes a HOTT element or "leads" (is merged with) a HOTT element (a PC/NPC may be integrated with a unit, like a HOTT general is part of a unit), they have Hits To Kill = their # of HD. Each time a PC/NPC has a kill scored against the unit they lead (or themselves if they are an individual unit), they lose a HTK. Upon 0 HTK, they are dead.
If a PC/NPC leads a HOTT unit that is destroyed and the PC/NPC survives, they become their own element.
PC/NPC individual elements function as regular HOTT elements for purposes of movement, flanking, closing the door maneuvers, melee, etc.
A PC/NPC may be designated as the side's general - a general does not have to be a PC/NPC.Easy-peasy. I'm still tinkering on any bonuses/minuses, and number of attacks, but I'm leaning towards Chainmail inspiration to say you have to be 4th level or above to add any bonuses to the Combat Factor.
Anyway, it was magic that gave me heartburn. I wanted the mages and clerics to be able to do their thing. Turn a unit of undead? Sure! Frag a unit with a Fireball? You betcha! Magic Missile? Uh... wait...
Magic Users and Clerics would not waste spells of an individual basis, like Magic Missile, unless they were in a man-to-man or skirmish battle (at which point, we'd move from HOTT to Man to Man and adjudicate the contest.) They would use area effect spells or spells that would influence multiple men. But, you may say... what about scale? Ah, that's a very good point.
HOTT makes no real distinction of how many men a unit ("element") represents. It could be 10, 50, 100 - the idea was abstract. So while a Magic Missile might affect a unit at a 10:1 scale, at a 50 or 100 to one, it's not really worth the effort. So when would *I* use HOTT instead of the normal D&D rules?
Well, HOTT says you create an army from 24 "points", and DBA (HOTT's Ancient Warfare parent) puts you at 12 units per army. So let's average that. The biggest battle I've run with my players is about 30 enemies and 15 PCs/hirelings. So a 20 or 50:1 scale feels about right... 12 * 20 = 240 creatures and I don't ever see myself adjudicating 240 enemies versus the players on a 1:1 scale, and at 50:1, we're up to ~ 600 guys in an "army" which I definitely would not adjudicate at 1:1!
So.. the magic missile. A magic missile taking out a single person in a 20 man unit is not going to seriously affect it all that much. So we can ignore Magic Missile... but what about sleep? A sleep spell taking out the max result of 2d8 HD affected could degrade/disrupt a unit consisting of 20 1 HD creatures.
Swords & Spells put the area of Sleep's effect at an inch (10 yards) and punted the specifics of the effects back to the spell listing in OD&D. In my mind, at @ 2d8 HD affected at 1", it's enough to hit a unit or two, although the Game Referee might have to be creative on how low you could go before a unit as a whole wasn't affected. Chainmail didn't have Sleep, but it did have Slowness and Confusion which affects 1 figure: 20 men (at the 20:1 scale - which the Fantasy Supplement is scaled at.)
The end result of all this comparing S&S/CM to my HOTT+D&D is that I have some metrics to help me in deciding how to "whittle down" what spells will truly affect a unit. In all honesty, CM is probably going to be the bigger help, although S&S does a nice job in ranges.
Now I just need to find some of my Dark Ages guys/gals who are brave enough to test some of this out...