Well… insofar as I understand the concept, dialecting in Rebol amounts to reusing the constructs of the language in new ways, to create context-dependent sublanguages for solving specific problems. As @hostilefork put it:
This is quite a different concept to ‘create your own totally separate language’. Rather, dialects are about giving different semantics to things which already look like Rebol programs. You can use dialects to help implement other languages (e.g. the Whitespace interpreter), but it’s not the same thing.
So, if you wanted to integrate YAML into Rebol, the natural way to do that is to add YAML as a new literal syntax, which you can then use as the basis for dialects. But I’ve already explained why that’s a bad idea. The alternative is putting stuff in strings, which immediately loses all the advantages of Rebol’s rich set of datatypes.