According to this site,
For pH values below 4.0, ferrous oxide (FeO) is soluble. Thus, the
oxide dissolves as it is formed rather than depositing on the metal
surface to form a film.
Once ferrous oxid (i.e. iron oxid) becomes soluble, your body is able to metabolise it. Your body is able to regulate iron levels to some extent, so while it doesn't seem likely that this, on its own, would lead to iron poisoning, anyone taking iron supplements might want to avoid cooking canned tomatoes in cast-iron. Fresh tomatoes have a pH between 4.3 and 4.9 according to this online source and should be alright.
Note also, from the site in my first link, iron corrosion increases drastically below a pH of around 3.8. If spiceyokooko is correct, and the pH value range for canned tomato is between 3.5 and 4.5, I think you would do well to measure the pH value of your tomatoes before using cast-iron. There are various inexpensive pH test kits and instruments available. It's quite likely that your pharmacy stocks alkacid paper or similar.
I also recommend you read this excellent answer about cooking wine and vinegar in cast-iron.