My two pennies:
I have one season (june till Sept) doing banner towing in Southern Spain. The companies involved are run very much in a friends only basis and it is difficult to break into them. It is however not impossible and it will involve the spending large ammounts of time at the airfield, getting to know everybody, doing errands, setting the banners, etc for free. Once they accept you you will need a vacancy to become available...
Another important factor, both for bannner towing and Crop Sp., is if you have any taildragger experience. Most of the former and all of the latter have taildragggers and nobody will let you experiment with their aircraft first. You will need to pay somebody to give you a taildragger conversion.
So Banner Towing is possible but not easy. There are a couple of firms in Madrid that will hire you for the summer and you will fly around the coast sleeping at a different location every night (quite romantic but also lonely). Other have a permanent base and you will do only ocasional trips away from the base.. .The flying itself is very rewarding and mostly a seat of the pants affair, you will learn to really handle the aircraft and to make decissions for yourself. Picking up the banner is great fun and requires quite a skill as you have to swop down and hook it from a set of small goal posts. It is true however that this is not procedural flying as taught in schools and you will have to set your own standards and keep them. It is all up to you.
Crop Spraying is a different matter. Here experience in taildraggers is even more crucial and they often require around 200 hrs tail experience. This is understandable if you see the size and power (650 hp +) of a turboprop airtractor and consider that you are expected to land in a small dirt track for refuelling and reloading. The main dangers are power cables, shody maintenace and ofcourse the chemicals in that order. Most accidents happen from hitting power lines and you are often required to fly underneath them in every pass to get the job done. This is very demanding flying and arguably not the sort of stuff to do with low hours. Accidents are very frequent and as a result the money is pretty good. As before it is up to you to set and maintain your own high standards and keep out of trouble.
My view is that if you like to do something like this, and the flying is superb, you will be better off getting in touch with gliding clubs or parachute clubs back in the Uk. I beleive that they do a lot of flying (no pay) and they are, relatively, easy to get into.
regds
cal