...well, easy to USE at least.
A 2 part solution
#1 is a UI/algorithm change: On the same screen as the UMHK when designating walls floors etc., have a 1-10 (using number keys) scale of priority you can choose as well for that block of designations. So I hit kkkkk and get a 6x1 line, and then also hit "1" and enter. Those will attempt to get built first, as if they were the only designations, and entirely completed before any designations are considered that were given a "2" or higher on the map.
Default is just "5" if you don't hit any number key, or something.
#2 is an under the hood only algorithm change: Whenever a dwarf is deciding in the code which designation order to complete first, they do the following:
1) Make a list of every construction designation that could currently be accessed (amongst those of the lowest priority assignments available, of course! See #1)
2) Amongst those, simulate building each one, and check whether doing so would prevent access to any of the others.
3) As soon as you find one where the answer is "no, it wouldn't," go ahead and choose that one to build. No need to continue checking the rest.
4) If there are no such options that don't already have a dwarf in the process of building them, then stop and check back later. No need to constantly calculate every tick if it's ready yet. It could wait until the next construction is completed on the map, to trip a flag for "okay ready to check again now everybody."
Combining the above two features would allow you to fairly easily designate huge constructions (many z levels of castle at once for instance) all at once, and have it correctly succeed in building them with no stranded dwarves or designations. For example, let's say I want to build a 10x10x10 cube of walls. EASY! Just U and K to make a 10x10 grid, hit priority 1, and designate the lowest floor. Then 10x10 grid hit priority 2 and designate the 2nd floor, etc. Then walk away and watch the dwarves build the whole cube with no further assistance, assuming they have stair access to all the levels. They might do it in very strange orders, but they will still do it, without getting trapped. Anything more than 10 high would require another round of designations, but come on, that's not a significant burden!
For more complicated things with, say, internal stairwalls, you may want to reserve some of the higher priorities for things like "build all the stairs first." You could also do this for a up to a 10x10x9 cube, if you reserve #1 for access stairs. The logic is pretty intuitive on how you would choose different patterns, and almost anything of a normal height can all be done in one single game pause.
The same system could also be extended to other designations like mining and channeling and deconstructing as well, of course.