I think that it won't work.
The reason is because most puzzles here have a single solution. In PPCG almost all of them have multiple possible solutions.
If you sandbox questions here, people could figure out the solutions before the puzzle is posted, and quickly post the answer, taking an unfair advantage and possibly spoiling its fun. Further, if the question was changing in sandbox before it is posted, people could get an unfair advantage by looking its evolution in the sandbox.
In PPCG this works (though it is really far from perfect) because the questions are competitions with multiple (normally infinite) possible answers. Even if you already know a possible answer, usually there is a lot of people which may have better answers. Moreover, knowing a possible answer there and starting to work in the solution before it is actually posted, rarely is an advantage. But here, this is normally a killer advantage.
And as per mdc32's comment, it is very hard to give any useful feedback about design a puzzle without knowing the answer.