I requested a couple of enhancements (#81 and #80) and I'd like to elaborate on what I'm thinking.
In fact, I think such changes could be made almost entirely in Spring, something like:
Better still would be if Thymeleaf had a more built-in solution. Maybe you can break off the Jackson-dependent part into a small thymeleaf-jackson project, which is accessed by, say, th:inline="jackson". Then the last piece is a hook somewhere to provide or configure the Jackson ObjectMapper.