I have never done this myself, but I would think that you could put the seeds in something that will hold them in place and then decompose rapidly. Maybe put them in a thin brown-paper bag, like the kind used for kids' lunches. Or wrap them in a couple layers of newspaper. It wouldn't take more than a couple rains to release the seeds from these.

Just noticed this thread -- there was a recent article in the Los Angeles Times about guerilla gardening accompanied by a photo section on making seed bombs... I hope this works.

Here is a link that might be useful: L A Times article

How about using those peat pellets?

1 part seed
2 parts gypsum
8 parts manure

mix the ingredients thoroughly while still dry. Add water slowly until the mixture is the consistency of cookie dough. Pinch enough of the mixture and form into balls the size of marbles, leave in the sun to dry for 2-3 days (don't attempt to bake them in an oven or the like).

Some insist on using powdered red clay but it's difficult to find where I'm at, gypsum bonds just fine. I've also heard of people mixing blood meal into the mix, another trick I haven't tried.

Native seeds are the best to use, wildflowers etc. Larger seeds (such as watermelon and sunflower) can take longer to germinate and are difficult to form. Look for drought resistant varieties such as california poppies if you can't tend to the "garden".

You can often find perennial wildflower mix in bulk so you don't break your budget.