One way or another, I think the solution will involve brighter lenses, in the sense of larger maximum aperture diameter, so as to deliver more total light to the viewfinder. (Aperture diameter determines this at a given FOV, independent of format.)
Two things typically change with smaller format DSLR viewfinders, so could you ask the optician which he thinks might cause the problem:
1) the image is smaller
2) the image is less bright with a given lens
The second is because the VF magnification is usually increased to reduce the first effect.
If (1) is a problem, increasing VF magnification could help; upgrade to a 20D, or get a clip-on VF magnifier or such. However, this makes the VF image dimmer, so changing to brighter lenses might be desirable.
If (2) is a problem, brighter lenses are clearly a solution.
the solution of changing to a larger format while still using lenses of the same minimum f-stop is also using brighter lenses in the sense of larger aperture diameters.
This need for bright lenses does not surprise me: the greater sensitivity of DSLR sensors allows one to get away with smaller, lighter lenses of smaller maximum aperture diameters, but our eyes are not getting any more sensitive are they? I have already sworn off using f/5.6 lenses in sub-35mm formats, except if it is all I can afford for long telephoto reach.