Just think about it: A legion of Hamutla teleport in, each carrying a massive double barrled supersoaker that fires Holy and Axiomatic water. It would decimate Tanar'ri forces! Supersoakers would be some of the best weapons used in the Blood War.
Tank/Firetruck hybrids would be the main combat vehicles.
I would think that a SuperSoaker filled with aligned water would end up functioning like an aligned weapon, so baatezu might run into trouble if their SuperSoakers become holy and they get negative levels.
But really, I'll give the same answer here that I did to the thread about why fiends don't run around with Uzis. Exemplar are formed from thought. Their bodies are literally composed of ideas and belief. So, I would contend that their forms should adhere to Jungian archetypes (very loosely, as most of the symbols involved here aren't mentioned by Jung). Fiends look the way they do because that's how people expect them to look. There's really no practical reason that they have to be scaly, ugly, and unwholesome to the eye, but mortals, in their deepest and darkest fears, imagine them that way, so that's how they turn out. The most powerful symbolism still lies with the sword, fire, the night, and looming winged figures. Guns and jet-fighters don't have the same depth of impression ingrained into the collective unconscious that these eternal symbols do, and so fiends just don't use them. They don't want to. It fills them with a certain weakness of conviction to do their dastardly deeds with a gun.
However, more to the point. Nobody would be afraid of a fiendish backyard game of squirt guns. It would be laughable and the power of the forces which the fiends represent would be diminished. Fiends have more important things to worry about than their direct, physical goals.
However, a canny planewalker, with a super soaker and backpack-tank filled with holy water... that's something I can see happening. Often. Especially among Xaositects.
You are right mixing the two can go to far, but there has to be some mixing, the things we fear can change. The pixies and fey that used to abduct people became aliens... the warrior scared of a ninja attack is now a guard worrying about a SEAL team. I allways saw fiends adapting to this logic, and while super soakers are a little iffy for one of their own aresenals, what about a vrok vareint which act's more like an A-10? I think it still fits the forces of evil, except this demon bird attacks by diving and strafing others with the powerfull roar of it's chainguns.
I can see the Blood War devolving into trench warfare, for two reasons: it is the archetype of wasteful, dirty warfare, and it has a strong emphasis on taking territory. I can see the feinds willing to expend millions of petitioners and mortals to gain just a few feet of ground, easily.
Oh, and a Holy Hand Grenade is entirely possible in d20 Modern. A set of 6 +1 Holy Fragmentation Grenades would be Purchase DC 30.
Just out of curiosity, but what would the armies of the Upper Planes be like? Troops, wepons, vechials whatever.
Since fiends rarely used bows and crossbows in medival times, I don't think they'd start using personal firearms in modern era. Most of them teleport at will anyway so there's no point in having an AK47 if the enemy can just appear in the claw range
As for artillery and tac nukes... :E
Hmmm... and interesting point.
While a gun doesnt carry the same symbolic weight as a glowing sword, artilliry might.
Forget the actual weapon for a moment. Consider the effect, esspecially with the giant, 'Big Bertha'-esq type artilliry. Explosions raining down from the sky, with almost no notice or defense.
These giant cannons might be more favorable to fiends then, say, a cruise missile. They are very similar to simple cannons, and I can easilly see the Bataazue creating fiendish new forms of ammo. Including, but not limited to, lemures in a can.
Think about it. Lemure are, lets face it, pitifully weak. Fire them inside a metal can at the other side, and they can smash up a fair amount of turf. Or, on a lucky shot, bang of a tanari pretty good. Lemure pops out and, at the very least, gets slashed to ribbons within moments. This is still moments that the Tanari spend not charging the Batazuue lines.
All in all, the co-ordination of aiming, firing, and reloading such a large and not very high tech weapon would probably make it more of a Bataazue weapon than a Tanari one.
And while fiends are typically immune to poison, maybe other types of gas? Acid wouldn't be too effective, but surly the loths would sell one side or the other certains bio-weapons. Even worse might be the failed bio weapons, both sides immune put still out there. That would make the Blood war even more frigthening. A Planewalker stumbles accross a simgle lemure on patrol, kills it, and walks away with 15 disieases the lemure didn't even know it had, and four it did.
what about holy water in a gas form that could prove to be useful or better yet axiomatic water/gas?
What about wearing a demonic/devilish wetsuit or a raincoat as armour?
And how are they getting a hold of holy water?
All I really know is I wouldn't want to be even in an "adjacent" plane duringt the intial arms escalation. The idea of the Nega bomb just scares the crap out of me.
And yet another random idea:
What about blessed napalm, it burns with the heat of bahamut's breath!