Maybe seeds isn't the way to go. A seed would be hard-pressed to contain all the nutrients to build deep enough roots.
Instead, how about one of the many forms of asexual reproduction common in plants, such as vegetative propagation. Basically, a branch of the tree hangs low, touched the ice, begins growing roots until it hits the soil, then the linking branch withers, separating the now-complete plant from its parent.
Or, scrap the soil idea entirely. As others have said, under so much ice for so long the soil won't be anything to write home about. Plants with their roots in water exist. Maybe your trees' roots melt the ice chemically and drink that up for water. Ice that formed through layers of snowfall won't have much in the way of nutrients naturally, but if your glacier hosts a rich ecosystem with animals and plants living and dying on its surface, it could work. That poses another problem though: either your microbes have evolved beyond the real arctic ones and can decompose the corpses in the icy cold, or your ice is less 'nutrient-rich water, but solid' and more 'sterile, unforgiving ice with pockets of preserved corpses', at which point your plants spread their roots (via chemical melt) through it to find and chemically digest the corpses - like carnivorous plants that eat with their roots.
That is more of a creeper plant than a tree, though, so you'll need an excuse why the whole bark-tree-thing is needed. And, don't expect them to be pleasant either: the thorns of plants like briars are meant to hook into careless creatures and keep them tangled until they die and fertilise the soil - right where the plant is, not way over there. If the ice tree had to crawl its way through the ice to find ancient corpses that might be there, it would either develop the means to create new ones where they're close and easy to reach, or form a symbiotic relationship with something that does. Your forest floor will be covered in preserved bodies or their husks, mostly those of desperate scavengers that came to feed on the previous ones and proved as unlucky.