Nope, I don't want to say it...
So it is possible for Maria Theresa of France to Marry George IV then?
I think so. It would take some work to be sure but it should be possible. For example, have the French Royal family's flight to Varennes succeed or instead have them flee to Britain. It would put Madame Royale and the Prince of Wales in direct contact and, once he decides to marry (which he will considering his debts) he could easily prefer a Royal bride whom he knows than one unknown.
I'll bet that, like the proposed match of Princess Beatrice and the Prince Impérial a century later, both mothers would support the match. The key issues would be the support of George III and the matter of religion. If the Kings of Britain and France can be brought to support it (IDK about what their feelings would be) then the religious question could be easily answered, either with Madame Royale converting or with a bill in Parliament authorizing the marriage, with Madame Royale remaining a Catholic. If its the later, it could still happen. After all, its been over a century sense England had a Catholic Queen (Mary Beatrice) and I know that the ruling classes did feel sympathy to the Bourbons (although how much I can't say, if Louis XVI's family is never imprisoned and executed), so there could be a bill to allow the marriage.