One thing I've learned from the forum is to never assume anything, so here's a question. If I live in Thailand, don't have a work permit, and work for a company outside of Thailand doing something like writing assignments or consulting work, would I be breaking any Thai laws?

Moved to Visas and Work Permits.

By the strict definition of Thai laws it would appear so as you would be expending energy inside Thailand.

Believe legal route would be a Thai company contracting and you being employed by them, with a work permit.

Any form of work in Thailand requires a work permit. But if working from home, and you tell no one what you are doing, you may well fly below the radar. But it is not legal. Being employed through a Thai company or setting up a Thai company structure is the legal route to follow.