Would it be possible/easy to support other fluid resources from other mods?
For example natural gas.
By a remote interface or a string list setting?

No setting needed. Just need the other mod (or a compatibility mod) to create a resource with the same mining category and collision mask as offshore-oil.

I am trying to fork, and port to 2.0, this https://mods.factorio.com/mod/xander-mod
It adds a fluid resource named "natural-gas", deep-copied from vanilla "crude-oil".

So, you will need to make a separate resource entity for offshore natural gas. Do a deepcopy of offshore-oil in data-updates, after depending on Cargo Ships. Then in data-final-fixes, set the offshore-gas "collision_mask" and "category" to deepcopies of those elements of offshore-oil.

You will also need to add the new resources to the Nauvis planet definition in order for them to appear in the map generation resource sliders.

Ahh super! So the oil_rig can pump all resources with category = "offshore-fluid" !
Thanks!