Can you ferment tomatoes in a fermenting crock with water seal?
Ripe Tomatoes

I've fermented ripe tomatoes, not on their own but in mixed vegetables. They get very soft. I used a mason jar, but I imagine a crock would have similar results. 

Note that this was a brine style ferment (submerged in salt water), not a kraut-style ferment (mashed with salt and submerged in its own liquid).

Personally, I won't do it again&mdash;but then I prefer ferments that are on the crisp side, so maybe it's a matter of preference. I feel fermentation works best with hardier vegetables, though it's possible that some salsa and chutney recipes may involve fermenting red tomatoes. 

Green Tomatoes

Green tomatoes, however, are a different story and ferment wonderfully. With garlic, dill, bay leaves, coriander, and black peppercorns, kosher green tomatoes are one of my favorite ferments. 

The only catch is that they can be hard to find, even at farmer's markets. But at this time of year (fall), I always have some left on our tomato vines, and I find fermenting is easier than fried green tomatoes. If you don't have tomato plants, maybe friends or neighbors do. 

"Undecided" Tomatoes

This is completely anecdotal and includes some guesswork, but may still be helpful:

This fall I put up four jars of green tomatoes to ferment. Three had major problems with kahm (benign yeast). Even though I aggressively managed the kahm, removing it at least daily, those jars ended up with an off-putting taste and I ended up discarding them... tears in my eyes.

I used the same procedure for all four jars that I've used several times before, But in the three failed jars I included a few tomatoes that were half ripe. I suspect, but don't know for sure, that the extra sugar in the ripe tomatoes made the kahm more likely. So it's possible that ripe tomatoes, as a relatively high-sugar item, could risk more yeast issues. 

Safety

I can only say that I've had no foodborne illness problems, and I'm aware of nothing that makes tomatoes more susceptible to these issues than other vegetables when properly fermented (enough salt, kept below the liquid, etc.).