I'm trying to make some copper block variants and I want them to act like regular blocks, so over time they oxidize, they can be waxed with honeycomb and can be scraped with an axe.
Now, doing the blocks and making them work this way isn't really a problem (you can see a WIP implementation here: https://github.com/JimiIT92/MineWorld/blob/master/src/main/java/org/mineworld/block/IMWWeatheringBlock.java), however

as you might guess I've duplicated the WeatheringCopper interface from vanilla Minecraft and added my own logic to make it work with my custom block.

That's because what I've originally tried is to add values to the NEXT_BY_BLOCK map inside the FMLCommonSetupEvent, however that is an Immutable Map, and as such I can't add values to it (I get an UnsupportedOperationException if I try to do that).
In a similar way I've also tried to add the waxed variants to the HoneycombItem#WAXABLES map, but again that is immutable.

So, is there a way to actually modify these maps so you don't have to rewrite the same logic?

Without trickery, no. There was a PR about a year ago, but it eventually went stale and closed.

Understand, well I guess is fine then to do my own custom weathering interface and replicate the blocks as I need. May I ask why the PR went stale? Was it discarded by Forge or it required more work and eventually got forgotten?

23 hours ago, JimiIT92 said:

May I ask why the PR went stale? Was it discarded by Forge or it required more work and eventually got forgotten?

A mix of both. The triage team wanted a different implementation of the PR to which the author never addressed, which eventually left it stale for a year and ultimately closed.