I finally figured out what I was doing wrong with assemblies [see The Wall Strategy for my dumbery there -- I thought mods had to be installed in order, which made things like PPTA impossible; that's what I get for using spoilers]. Now I'm into mod packs. So here's my first moderately successful Ao100 Scavenger run for your amusement.
This is something like my 15th attempted Scav Ao100 HMP run, the others died due to insufficient ammo and firepower. This one was lucky enough to find an early Lava Armor and a Nuclear Plasma Rifle just after the Deimos levels, and that made the difference.
I've been cobbling together all the assemblies I could to produce the common mods I need [several Elephant and Gatling Guns]. I'm currently using Cerberus Phase Boots (A) [note that their +15% move speed does NOT carry over onto the assembly!], Cybernetic Armor (PN, hoping for an O+P just for fun) [Cybernetic Armor is the best thing a Technician could find, right? Right...?]. Weapons are Burst Cannon Nuclear Plasma Rifle (no mod yet), Hyperblaster (F1), and a very very lucky find: Demolition Minigun (F1).
Belt-fed grenade launcher. Yes. With Eagle Eye (5) and Son of a Bitch (5), it will gib a Baron in one burst, and kill most anything that was next to him. It's a real treat.
Apparently a Burst Nuclear Plasma Rifle (S1) on this trait build will also toast Barons and Viles at mid-range in one burst, but the damage varies wildly. The satisfying overpowered feeling is somehow also gone, even though I'm often shooting through walls and killing things a screen's distance away.
Strolled across a lava river on level 85 with my new antigrav boots on and died. It was only four spaces... Kinda glad it's over anyway, the difficulty wasn't scaling up with my arsenal enough to hold my attention. Final opinion: Scavenger is crazy hard to get rolling, but ridiculous if the RNG gives it a chance to start snowballing up.