The phrase 'more seldom' is correct and usable in written English.
You can use it to make comparisons between two things that don't happen often. For example, "The northern lights are seen more seldom than the aurora borealis."
He came more seldom now, but was able to stay longer.
A person's path through the world seldom changes and even more seldom will it change abruptly.
And even more seldom do they resolve their differences by sleeping together.
For this reason rock falls, rock slides and, more seldom, topples are very frequent in the area.
It is a good question, seldom asked by those who make American foreign policy, and even more seldom answered.
I rarely read books that other people give me, and even more seldom cajole others into reading books I like.
They often move in vast herds of 500,000 animals or more, seldom stopping or pausing because they must constantly forage in these environments of generally low productivity.
Only rarely, nowadays, are books banned; more seldom still are they burned, and the sight of flaming pages, as with "The Satanic Verses," means trouble flaring ahead.
It's one that is more seldom seen than Godot, but which, like that play, features towards its end a mysterious small boy.
In current practice, many imaging methods are designed such as special sequences of MRI (dynamic susceptibility contrast perfusion and susceptibility-weighted imaging, diffusion), proton magnetic resonance spectroscopy, positron emission tomography, or more seldom 201-thallium single-photon emission computerized tomography.
What he/she is doing now are much more seldom activities, related to risks and heuristics.
