As per Steve's response from this morning (I'm following his blog at sabot03196.wordpress.com and he is very kind to respond to my comments with hands-on advice), it looks like I'm going to need even more than 10L of extra fuel I already packed.
It appears that I missed the part where you have to take a 95Km detour to Cartwright in order to get gas between Goose Bay and Port Hope Simpson. It would extend my already long day by almost 200Km and I probably wouldn't be able to make it to Blanc-Sablon for the night. I don't think I can ride 800Km of unpaved road in a day:
So, instead of buying another jerry can that I probably won't need again I decided to bring a 5.5L laundry detergent jug. I've used it to store gasoline for the lawn mower with good results and I can throw it away once I cross Labrador.
Alternatively, I can use a 4L kerosene container I have. Even better.
I just checked my numbers again. Google maps says it's 405Km between Goose Bay and Port Hope Simpson. That means that 14L (tank) plus 10L (can) will get me there if the bike doesn't burn more than 6L/100Km. I'm almost certain Majesty can achieve better fuel economy than that even fully loaded and on loose surface, but I'm still going to hedge my bets with another can. Unlike the one under the seat, that extra one will be strapped to the top case rack and kept empty until Goose bay.