I read your piece about pen names last week with interest. I have been toying with the idea of using a pen name because I am currently contracted to a publisher I'm not enjoying working with. The contract I signed gives them the right of first-refusal on all my future books, but I don't really want to give them any more of my books. Can I use a pseudonym to query agents or submit a new book to different publishers?
Please do not do this. It is dishonest and will likely land you in a world of legal trouble further down the track.
It sounds to me like you have a bigger problem here than choosing whether or not to use a pen name, and rather than trying to avoid that problem, you need to face it head on. If you're unhappy with your publisher, you need to let them know that. It may be that by opening the lines of communication, whatever your issues are with this publisher, you may be able to resolve them.
If not, then you need to terminate your contract with this publisher. Only once this has been done can you offer your books, whether written under your own name or a pseudonym, to another publisher.
Using a pen name to try and avoid fulfilling a legal contract will never work because even if you do write and publish under a different name, any contracts will need to be signed using your legal name. Royalties will be paid to an account associated with your legal name, even if you set up a separate 'trading as' account for your alter ego. Even legally changing your name to match your pen name (which is a fairly extreme step to take) will leave a paper trail leading back to you.
Better to be honest and upfront with your publisher, get your grievances out in the open and move on with your career and your reputation intact. And your name, which if you have already published books, may have a following you don't want to lose.