Adding an architecture is a very doable job, and the MIPS is a really friendly architecture to implement. It is hard than an OS port though, since you need to get at home with the compiler architecture. The majority of the work to port to a new processor is implementing all abstract methods of the cgobj unit and overriding some parts of the front-end code generator. Because the MIPS is a very regular architecture, overriding the front-end code generator will be unnecessary in most cases.