This feels kinda unfunny: Was fooling around with the Tacens OrumX, which I ordered a few months ago. Just dropped the be quiet SL 2 280 inside. Yes, fits, although you need to wiggle a bit.
Then I thought: Wouldnt it be funny if the GPU would fit side-by-side, too? Just like as planned for the Meshi?
.. turns out: Not only does it fit, you could even get a regular sized (= regular width / height) GPU inside. 
Max height approx 310ish mm or so. And if you wanted to keep using the PSU location on top, you'd be limited to about 240 mm length. So most RTX 4070 and also several 4070 Super models would fit.
Now get the Meshlicious / Meshroom S middle frame, cut it down at the bottom by approx 4 cm, add 90° angled brackets to it, and tap the case bottom. Et voila: Mounting option for the GPU achieved.
Add one of the mounting brackets for the Meshi or the Meshroom S, cut this one also a bit shorter, just so it fits snuggly within the case top and bottom, and then mount the radiator to it.
Additional tasks: proper ventilation - eg. cut the existing side panel and add some mesh, preferably of the same color as the rest of the case (I got the white version; so for this course, the black / dark grey version would've been better).
Et voila, 14L mATX (!) SFF PC, liquid cooled, with ~2kg weight of the case itself achieved. 
cu, w0lf.
