Doggonit,
There are many ways to carry extra shotgun rounds. On my home defense gun, I keep a nylon 5 rd carrier on the buttstock filled with some extra low recoil buckshot. I have been thinking about getting either a 4 or 6 rd side saddle also, perhaps to use for slugs to keep them seperated from the buck. While awkward and a little hard to get use to, they aren't all that bad once you get used to it being there.
The bandoleers you mention look cool and any young Rambo-ite would be happy with one. But from what I have seen, the shells are a bit difficult to get at in these things. I am more accustomed to reaching down for spare ammo instead of up. But they do offer two advantages and that's quiet carry as well as the ability to carry a large number of rounds.
If I wanted more than what was in the tube and nylon carrier, I would either strap on one or more of the 12 round shotshell pouches available from Cheaper Than Dirt. Or you can do as the troops did in Vietnam and just cram a bunch of shells into standard US mag pouches. They will hold a ton that way, but are less quiet than some other methods. In an emergency where you don't have time to gather pouches or other special gear, just fill your pockets with shells. Beats nothing and is fast.
As far as less lethal munitions go, I have mixed feelings. I think they are a wonderful resource for police but I also feel they are less practical for civilians. Cops usually have a partner or would have several back-up units on scene or enroute to any situation requiring less than lethal or lethal force. They also use these rounds (such as bean bags) from specially marked shotguns. These are often kept in the trunk and clearly marked with bright yellow or orange stocks and forearms. In a civilian defense situation, you are likely going to be alone and trying to use lethal and less lethal ammo from the same weapon could present serious problems if you get confused. My idea is that if the situation isn't serious enough to use live rounds, then get out of the situation. As civilians we have that option. Police have to affect some type of outcome when called to a scene where the suspect is clearly a threat/danger. But in many of these cases, lethal force would not be justified at that instance. The police don't have the option of walking away. But civilians do. Consider this before deciding on less lethal munitions. Do you have someone to cover you with another weapon in case the less lethal rounds are ineffective? Do you have a spare gun that you can clearly distinguish as being loaded with less lethal munitions? If you answered no to either or both of those questions, I would avoid them altogether. But you can still supplement your carry gun and or shotgun with CS/OC pepper spray and perhaps even a folding knife. That will give you a more complete defensive package, yet not create any additional confusion.