Would it be possible to add basic CraftTweaker support by registering the mod's RecipeTypes? This would allow users to add and remove recipes in a much more convenient way compared to having to make custom datapacks.
For your convenience, this is how the Create devs do it:
https://github.com/Creators-of-Create/Create/blob/mc1.18/dev/src/main/java/com/simibubi/create/AllRecipeTypes.java#L82

@jamesjimmy88 IC2C doesn't have "IRecipeTypes" for Machines. And the Crafting system for Vanilla is insanely slow for what it can do. So we won't add recipe types.

You don't need Crafttweaker support to make things work, ic2c does support Datapack recipes just fine :)
Also if you want Crafttweaker support @Trinsdar was making it for 1.12.2
So he is the person to ask :) (Sry trins)

Is there any way to customize recipes for the 1.7.10 version of the mod?
It seems that the plugin is only available for 1.12, and the .json system was also only implemented after that point.
Datapacks naturally didn't exist in 1.7.10, either.

I'm particularly interested in using the machines specifically to process some items.

@DarianLStephens
You can use CraftTweaker and ModTweaker mods to add/change IC2 Recipes, as it was supported back then.
Unfortunately, the old wiki for CraftTweaker/ModTweaker 1.7.10 is gone, so no info can be found, but you can take a look at ModTweaker GH page, where you can find supported machines.

Here is an example of Macerator recipe:

Pattern: addRecipe(output, input)

import mods.ic2classic.Macerator;

Macerator.addRecipe(<minecraft:diamond_block>, <minecraft:dirt> * 64);