Well, MTF is a property of the lens; not of the input scene, and not of the output image. The lens' MTF is not a single function either. Although normally plotted for a system in perfect focus, it changes (worsens) as the PSF changes on either side of focus. So, bringing DOF into things isn't really fair; because the scenario you gave ("a smaller format at a given aperture") gives an advantage to the smaller format system thanks to its greater proportion of scene convolved with a (nearly) in-focus PSF - but it doesn't deliver the same
picture due to the DOF differences which would be clear to the viewer.
To equalize the pictures, requires equalizing the DOFs, which means stopping down the larger format lens more. Then one gets into this familiar argument:
"equalizing the DOF -> stopping down more -> more diffraction -> blurrier PSF -> detail lost in direct proportion to the format size -> conclusion: all sufficiently well sampled formats deliver the same detail for a given picture; there is actually no point in using larger formats".
Where that conclusion falls down is that it assumes that all lenses are diffraction limited at all f-stops; that for smaller and smaller formats, there will exist faster and faster lenses which are still diffraction limited wide open.
In practice, we are much more likely to find that our larger format lenses - even cheaper old ones - are happily diffraction limited at their modest and small f-stops than the smaller format lenses are at their wider f-stops; thus delivering the same picture DOF but with higher levels of in-focus detail.
Flat planes...or the inside "surface" of an infinite sphere (astrophotography! :chug
...or if it's a non-flat-plane nearby subject but you are not looking for deep DOF, and just want lots of resolved detail where it is in focus.
Well, sort of...if you choose to use a certain pixel pitch to normalize everything. But the standard units are lp/mm spatial cycles, which has the advantage of being independent of pixel size, or indeed of any kind of sampling. MTFs existed prior to digital imaging!
Ray