Well, there already exist peat plant pots. You plant a seed or cutting into it, the seed grows, when you see the roots protrude, you bury the pot. The plant continues to rip the pot apart as it grows, its earliest roots are protected from the surrounding soil.
So lets see what we have so far. You start with previously recycled paper, of course. The recycling process results in some shortened fibers, so it will have to be thicker. You don't bleach the paper, so the fibers will be more visible. Now you have an odd looking poster stock. You use soy inks, which may lack some vibrancy, but are perfectly biodegradable (I guess, I don't really know.) So this is a kinda tacky looking poster, but that's cool, now it has an eco-hippy-look for extra cache. You embed a seed, and let people know when the poster is done with to plant the poster. If the poster isn't too big, they can bury it, and the plant will grow.
Your TiO2 plan, as I understand how it works from some information I can Google, will only oxidize dispersed particles, while the TiO2 is itself dispersed as nanoparticles, and only under intense UV, not while buried.
As for a plant nutrient. That's a kinda tough one. First of all, you can't store much. Second of all, exposure to water will cause it to leach away, if the nutrients are very soluble. A sprouting seed doesn't really need fertilizer, and the tiny amount you include will be gone by the time its really needed. You might as well for forget that bit. Its really enough that the paper will help hold water and provide some extra porosity to the soil.
You might want to consider the eco-consiousess carefully. What plant seed are you going to include? Just about any plant can be considered "invasive" depending on the locale.