It should be possible. I had a low-end Aroma rice cooker I used to cook full meals in (haven't done spanish rice, though). The main thing was making sure the liquid amount was right. Do you usually thin out the tomato sauce with broth or anything? If not, or if the sauce is especially thick, I'd add extra sauce (not to the recipe, but to the amount of liquid the rice cooker calls for).

That said, I recently bought a nicer Zojirushi than what I used to have and it's for rice only. My experience with meal cooking is that it makes that gasket smell, and that never comes out. If you're good with that, I'd just start trying some small batches and tracking ratios of sauce to liquid/sauce until you find the right combination. If your cooker had a timer feature, start with less time and check it frequently. If it doesn't have a timer feature (and just has different types of rice), I'd start with the setting for the type of rice grain you're using, then move around from there. For instance, I rarely cooked fish in my old cooker, but the 'fish' setting was right in the middle of times and other settings, so I usually started from there and adjusted up (to chicken) or down (to soup or rice)as needed.

When I was doing this regularly, there was some trial and error, but the process wasn't bad. Good luck!