Whilst this is a valid request and quite a few people could benefit from it I believe that It wold be a massive undertaking and that the sheer number and variation of questions here would make it not feasible.
There would still be so many questions dealing with a language or some library that would be unlikely to be implemented into such a "code preview" feature.
What you might consider doing, if you come across a user or a question that could benefit from such a feature, is
- comment on the post bringing to the OP's attention the existence of such tools.
- suggest to them the possibility to post some sample (working) code using the aforementioned tools.
As was said in the comments above, It is desirable to have the "raw" code snippet in the body of the question itself - This allows users who are attempting to answer the question to quickly see the code that is being used and possibly spot the issue immediately - sometimes all you need is another pair of eyes to spot the semi-colon you are missing :P
Users also sometimes are deterred from questions requiring them to navigate away from the site in order to simple understand the question.
So - even if you do suggest to someone to use the online code previewing solutions you should also recommend to them to include some (if not all) their code in the body of their question.