Hi Björn,
We unfortunately don't have an official way to order fields. Technically you can do it by modifying some internal state: The field_classes list on the fieldset you're registering in. Or, to order fieldsets, to the _key_order list in the fieldset registry (reviewboard.reviews.fields.fieldset_registry). Untested, but if you want to try messing with that, you can. We probably should add something for reordering or adding into a position.
If you want to just modify the description instead of the fields/fieldsets, there are some signals available, but maybe not an ideal one depending on when you want to modify the description. We've been discussing adding extension capabilities for better description modification when bringing in commits.
Can you let me know what source code management solution you're using? There are some differences in how we bring in initial descriptions based on those.
Christian