Is it considered bad form to post an answer inside another thread asking a related question to the OP's original question? Sometimes I would like to leave a question as a comment, but being a Stack Overflow newbie, I don't have the rep yet. Perhaps posting a question as a comment inside a preexisting thread is also bad form.
Yes, it is "bad form". As your language acknowledges you're considering posting a question as an answer. This is a contradiction, and should therefore be avoided. Just a note, Stack Overflow and all Stack Exchange sites are not forums; they're question and answer sites and there are no "threads" - there's questions and answers.
If you need to ask another question then please ask one. Build on the question that you want to reference, acknowledge it, and explain what extra you want to do including your attempts and what you think you're doing right and wrong.
Commenting only requires 50 reputation, that's one good answer or 25 decent edits. It should only take you a few hours and in getting there you'll have given something worthwhile to the community. If you need to comment make that contribution and you'll be able to do so.