I see you saw my post about 3d printed spacers. I hope it will be useful for you.
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I probably won’t print any spacer as I need my brake but I found your solution clever so I liked it 
You can use bigger spacer + bigger brake disk.
Or spacer + different offset spacer for caliper.
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Sure. But I don’t think it will be necessary. Thanks!
Two weeks later: I have ordered and received a Maxxis Reckon+ 27"5x2"8. It fits perfectly into my frame and onto my rim. There’s about 2 cm of top-clearance and 1.5 cm on each side. A 3" tire should fit but there wouldn’t be much clearance left then.
Next step: try this new tire. I haven’t had much time because of the current curfew here in France but I should manage to find some time during the week-end to try it
In a 27,5" qx frame, can I ride a 26x4 wheel? What is the biggest wheel you tested?
Maybe I’m not using my brain, but I don’t think the tire would fit in horizontally (as above, Maxence said a 3" tire would barely fit, so I would assume a 4" wouldn’t).
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Well my brain said that if Maxence had 1,5 cm of clearance on each side using a 2.8" tire, a 4" tire on a wide RIM would be near 0. But NEAR means that I’m not sure if it is 0- or 0+ considering that 4" comes in wide range of air volume. It should rub the frame only when mounted, but maybe someone can confirm it
If you are pedaling hard uphill, 1cm on each side is not that much clearance.
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In my 27.5 with 3” tire I have only 2.5 mm clearance per side.
I have not experienced any tire-frame rubbing when climbing. And I put on a lot of force because I have to power 100kg against gravity
Ok I will test as soon as I get a used 26" fat tire
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Hi there! Yesterday I have tried to put my 27"5 x 3" wheel in my Qx frames and it fits. There’s still enough space to ride without issue
The tire is a Kenda Havok and the rim is a traditional Qx-Series rim. I doubt we can go bigger but it’s certainly already wide-enough for all type of riding 
Is the frame stiff enough to not rub the side of the tyre when pushing hard up hill?
How much clearance do you have?
I have not tried to go up hill. That’s was just a test for my freewheel brake I told about in Brake creaking - #6 by mowcius.
Here are some pictures I took of my set up:
I’d say there is less than 1cm of clearance on each side.
That looks pretty good. Enough space to not worry about the tyre catching and locking up.
Sure. But I’d say the Kenda Havok is a “little” 3" tire. You may expect other 3" tires to not fit.
The tire hitting the frame is really a non-issue:
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