TOPIC:

I would like to get my tank off and do some cleaning, maybe get into the carbs, but at least replace a fuel filter. There’s also a California emissions unit that I want to remove eventually, so I need to plan for that. But being a constant vacuum type petcock, there’s no off position, so how do I get the tank off without leaking fuel everywhere? This kind of feels like maybe a dumb question, but the FSM doesn’t make it easy to understand. Draining all the fuel via the carb float bowls seems like a pain in the ass, and like it would require a lot of hose that I don’t have. Or pinching the fuel lines might be easy if I could get some clamps on them, but I don’t want to do any permanent damage. So what am I missing here? Is there a trick that I’m not thinking of? How do I get the tank off without spilling gas/petrol all over my bike? 

Normally, constant vacuum type petcock stays shut in ON and RES positions unless engine runs or other source of vacuum connected to vacuum port. If such petcock leaks without vacuum, well, it's time to replace it or overhaul it. In case of overhauling, use as many OEM parts as possible. For all my experience, aftermarket fuel petcock kits at best work 50/50. Personally I prefer to replace petcocks with new OEM ones. It isn't cheap, but  one doesn't do it every other day.

Like gazzz said, it shouldn't let any out unless it's on prime. To make sure all the fuel is out of the hose before disconnecting from the petcock, I drain the nearest carb and leave it open, then slowly pull the hose off of the petcock, breaking the "seal" at the top first - so then most of it flows out through the carb. 

There is no fuel filter in the system other than the petcock has a built in screen around the pickup tubes.  If you decide to install a filter make sure it's designed for a gravity fed fuel system like a lawn mower.  Automotive type paper filters need pressure to flow so stay away from those.  

When the engine is off & the petcock is set to either ON or RES, fuel should not flow.  If it does, then the petcock needs service. 
You could disconnect the fuel line then cap it >> the port is probably 5/16.  Or disconnect the fuel line & connect drain hose to the port.  

Thanks for the help all. I will try to get to this service soon and will report back how it goes.