I have a special hose attachment for my fuel bladder.I've never used the fuel bladders, but experience with filling flexible camping water bottles, camelback packs, wine storage bags, etc. makes me skeptical of the ease with which fuel can be filled and emptied from these bladders vs a rigid can. I'm not saying the bladder can't work, but I can picture a clumsy moment resulting in a fuel spill mess.
Wise words there Mud!!For that Dempster Highway tour in Yukon and Northwest Territories, consider this.
Roughly 500 miles from Dawson to Inuvik with only one fuel station roughly half way. Average adventure bike can do something over 250 miles but nowhere near 500 on mud and gravel. Then there are road washouts, fires, other unforseen things that may necessitate turning back at the 200 to 220 mile mark. You run out of gas on the return. I am being rough about actual miles, but I hope you get the idea. Out in the boonies you need enough fuel to get there and back just in case.
It is always a good idea to plan for these contingencies before you depart.
At least bring a syphon hose so you can beg fuel from a passing motorist.
But perhaps not road legal in many civilized areas of the world.Do your own research, but PET and HDPE bottles can hold gas for a few months without collapsing. Use temporarily for those occasional long legs without fuel and discard once you're done.
Petrol in glass and plastic bottles is a ubiquitous sight in Bali, thailand, etc.
The other alternative is trip planning. Instead of the dozens of hours spent researching, saving up, buying, installing, and fiddling with an aux fuel setup, you could just take that leg of your trip at 50 mph instead of 80 mph - with panniers on, that works out to 2.9L/100km vs 4.5L/100km, or 480km range instead of 310km range.