Got a super hacky workaround, if anyone needs it in the meanwhile. Code + explanation is in the mod folder.
TLDR: use a "base" spot-noise with constant values, then apply all scaling etc. on top of that. Except there's like 3 major issues with that that required their own workarounds. But in the end ...

With this code in a data.lua (no other code anywhere else, no other mods. zipped mod is attached.):

local copper = data.raw["noise-expression"]["default-copper-ore-patches"]
copper.expression = copper.expression
:gsub("regular_rq_factor = 0%.11", "regular_rq_factor = 0.03")
:gsub("regular_patch ...

Huh, I've never noticed that it's possible to turn off a resource before .

Anyhow, when I turn off Iron, I get small circular copper patches, roughly of the dimensions shown in the screenshot.
If instead I turn off copper however, I get perfectly normal iron patches.

You should use some regex for the string which will be replaced. In the base game, the rq values are 1.10, not 0.11.

Whoops, forgot to escape te . , tho that shouldn't change anything. resource-autoplace.lua resource_autoplace_settings divides the
regular_rq_factor_multiplier by 10, so 0.11 ...

Edit: It's now tracked as a bug: https://forums.factorio.com/viewtopic.php?t=131528

My initial goal was to have a small resource patch in the center(-ish) of another resource patch. I figured I could make them use the same location by ensuring that their spot_noise expressions use the same skip ...