Pseudoliquids would be materials that DF could handle as if they were both liquids and stone. Meaning that dwarves have to dig through them. But they spill out and have layers just like water. Stuff like sand could be handled this way. They flow very slowly and stop flowing when adjacent squares have just one level of difference in liquid-height (ie. neighboring cells with 1/7, 2/7, 3/7, 2/7, 1/7 sand levels would not flow), which would also stop the back and forth flowing seen in water filled squares. Dwarves could walk, albeit slow, on 1/7 ans 2/7 sand filled squares, but would have to dig thru 3+/7 squares. Dug sand squares could be turn into compacted sand stones that could be carried away. Mud could also be handled as a pseudoliquid which doesn't flow or flows much less than sand.
Maybe snow could be handled like a liquid but without the flowing. This way, snow accumulation could be implemented on maps where snow storms happen. Snow would have to be removed thru digging. Still snow wouldn't be ice, and it mostly would evaporate directly instead of turning into water, or generate very little water (ie. 7/7 snow turns into 1/7 water), after being exposed to heat or reaching spring.
Pseudoliquid stones (compacted sand, mud or snow) could have some uses but it would be probably impossible to build structures out of them. They would probably turn again into pseudoliquid squares when dropped just like ice stones turn into water.
Finally. This doesn't have anything to do with pseudoliquids I guess but glacier maps should have some sort of icecap regeneration mechanic. Although I have no idea how this should work as I'm not so sure how real icecaps form in the first place.