It is not solid solution which I asked for, but since there are no better answers at the time of writing this should fit.
If you spot here an error or over-simplication let me know.
I will assume tire is torus-shaped. It is not true, but it has to stay, because real shape is to computational expensive to handle.
Like crossing the rails, here diameter matters.
26 + 2 * w26 = 28+2*w28
So you get
w28 = w26-1 (translating into English -- you can have width 1" narrower than with 26" tires to get the same effect of rolling over holes).
Another assumption -- when we hit an obstacle only pressure is changed, and the force from the tire material is negligible. Not even going to formula for force of changing pressure of the air, it is sufficient to notice we are not seeking the formula, we are seeking the same effect for 26" and 28". So the only factor that matters here is the volume of the tires -- it has to be identical to get identical effect.
Volume of torus:
2 * Pi ^2 * R * r ^ 2, here
r is half of the width of the tire, and R is half of the size of the wheel.
2 * Pi ^ 2 * 26"/2 * (w26/2)^2 = 2 * Pi ^ 2 * 28"/2 * (w28/2)^2
26" * w26^2 = 28" * w28^2
sqrt(26/28) * w26 = w28
The scaling factor is here ~ .96, this is more tighter constraint than the first one.
For above 2" width tire (my case), I can have 0.1 narrower tire in 28" to get the same feeling when riding.
If I want to ride comfortably I have to consider air resistance.
resistance ratio = w28*(28+2*w28) / w26*(26+2*w26)
It tells how much resistance will change:
= sqrt(26/28) * w26 *(28+2*sqrt(26/28) * w26) / w26*(26+2*w26)
= sqrt(26/28)*(28+2*sqrt(26/28)*w26) / (26+2*w26)
= sqrt(26/28)*28+2*(26/28)*w26 / (26+2*w26)
= sqrt(26*28)+(52/28)*w26 / (26+2*w26)
=~ 26.98+1.86*w26 / 26+2*w26
For 26", 2.35" tire the ratio would be 1.02.
Please note there is constant speed assumed. You usually go for 28" tires to get more speed, and air resistance is proportional to the square of the speed!
Now, for gusts of the wind from the side:
I don't count for spokes, fork, they are important factors, but it will make computations even more complex (and besides, the effect will be different for blade spokes, and for regular).
= (14+w28)^2-14^2 / (13+w26)^2-13^2
= (14+w28+14)(14+w28-14) / (13+w26+13)(13+w26-13)
= (28+w28)*w28 / (26+w26)*w26
= 28*w28+w28^2 / 26*w26+w26^2
= 28*sqrt(26/28)*w26+(sqrt(26/28)*w26)^2 / 26*w26+w26^2
= 28*sqrt(26/28)*w26+(26/28)*w26^2 / 26*w26+w26^2
= 28*sqrt(26/28)+(26/28)*w26 / 26+w26
=~ 26.98+0.93*w26 / 26+w26
w26=2.35. The ratio is 1.03 (a bit of surprise).
So, there is no free lunch after all ;-) However if you forgive discomfort when hitting obstacles, you can build 28" bike faster in regard of air resistance and yet the one which rolls over with much more ease than 26".
Because of the difference of the radius, even the same weight is more harder to to move in case of 28". I will calculate this factor some other time.
If I go for 28" bike (I didn't yet decide) I will change my 26" 2.35" tires to 28" 2" -- they should be faster (~0.9 less resistance), they will go more easily over the holes, and they will be just a bit stiffer when hitting an obstacle.
I thank Slovakov from BikeForum.pl for spotting the errors in equations. All still existing errors are only mine of course ;-).