The charging bit is not easily done. Most bicycle dynamos / dynohubs produce 6 VAC (the actual voltage varies depending on speed; if they follow the German StVZO requirements, they produce at least 3 watts). The nominal 6 VAC is relatively easy to rectify to 5VDC for a USB charger, but in order to charge a 12VDC battery, you would need to rectify and step it up to 13.2 VDC. You'd most probably have to build the circuit yourself as I do not know of any off-the-shelf devices that will do this (there are IC chips that will do this, but you'd have to build the circuit yourself).

What you could do is to charge a 5VDC USB lithium-ion battery pack (using a variation of the [USB Bike Generator circuit][1]) and then use a 5VDC->12VDC step-up to power the horn (or to get a 5 or 6 VDC horn).  It would be much smaller and lighter than a 12VDC lead-acid battery. 

Alternately if for some reason you're set on lead-acid, you could get a 6VDC lead-acid battery. The charging voltage for these is 6.6 VDC so it'd still be a bit tough to get your dynohub to charge it safely and efficiently, but it's doable. There are as I said, 5VDC horns.

Resources:

* http://www.sheldonbrown.com/dynohubs.html
* http://www.peterwhitecycles.com/shimano3n70.asp
* http://swhs.home.xs4all.nl/fiets/tests/verlichting/stvzo/index_en.html


  [1]: http://m.instructables.com/id/USB-Bike-Generator/