Well, what I wanted to do was exactly what I said. Modify the drop rate so that systems and turrets drop more or less frequently. I didn't want to modify rarity or quality. I haven't touched the game in almost a year, so I'm really missing the old xml files where I used to be able to tweak drop frequency based on block destruction.
I was reaching the same conclusion you did, that it's now hard-coded somewhere and no longer moddable, but thought I'd ask to be sure I didn't miss it somewhere.
Looks like I have to modify the tables for all rarities and see if that works.