I'm interested in contribution, and not really sure where to ask about this (in general, projects have a CONTRIBUTING file for direction on this).

I also have (a) suggestion or query about something you may already be planning (specifically: Are you planning to implement the ability to (de)select which blocks it's possible to grapple on to), where is best to ask about this kind of thing in future?

We can't label issues (that's a permission you have to grant us, like being able to write directly to the repository).

Hearing that's a gamerule is cool, how does that work though, if I run that command does that mean the only blocks I can grapple are cobblestone and stone?

/gamerule grapplingBlocks minecraft:cobblestone,minecraft:stone
means that it will only attach to cobblestone and stone.

Just make an issue on github and label it enhancement/question.
As for the ability to select which blocks it's possible to grapple on to, that functionality is already implemented.
Just use the grapplingBlocks gamerule, for example:
/gamerule grapplingBlocks minecraft:cobblestone,minecraft:stone