Maybe a dumb question, but... I'm a Canadian who would qualify for a TN visa if I worked in the States, but don't currently have a visa or green card. On the online submission form for job openings, it always asks "Are you legally entitled to work in the US?". Am I meant to answer yes or no to that?

Unfortunately, the correct answer is No because until you have the TN, you are not legally entitled to work in the U.S. Of course, this means that you will be excluded automatically for consideration of certain jobs.

Not a lawyer but have been working in the US on a TN status for multiple employers and I would answer yes and discuss further with the recruiter or hiring manager.

TN status is conceivably something you could get without help from the employer (though even the smallest startup hired one to help me set up a package). What you need is basically proof that a company wants to hire you (offer letter), evidence that the company exists, and proof that you fall in one of the TN occupations.

Also not a lawyer, but wanted to second what jsbg said.

I've been through this process many times, and I would always say yes and then mention with the recruiter (even if I had to explain that the process was basically "asking politely for permission at the border").

The underlying question that employers really care about is "will you be legally allowed to accept a job offer without unexpected expenses or delays". Even if you file through USCIS (and not petition at the point of entry), you can (should) have an answer in 14 days and ~2k in fees, which is 1) a drop in the bucket for any hiring budget, and 2) not impactful to a hiring timeline.