Inversion or fronting with "so"
Solution 1:
These can be analysed either as instances of Subject-dependent inversion, or as Subject-postposing. One reason for choosing the latter is that postposing of the Subject is entirely optional. Notice that in the Original Poster's examples the two instances of so are quite different. They are both used as anaphoric pro-forms, but are being used to represent quite different types of phrase or clause which both have very different grammatical functions:
So says the preacher.
So began the fight.
In the first sentence so is the Complement of the verb say and represents a content clause. In (2) so is an Adjunct meaning something like in this way. When anaphoric so is preposed like this, we can also postpose the Subject. This is optional as can be seen from the examples below:
- So the preacher says.
- So the fight began.
(Notice that so must be stressed in the last example - probably to distinguish this usage from the usage of so as a connective or conjunction.)
The grammatical status of the proform so makes a big difference to the effect of postposing the Subject. When so is the pro-clause Complement of the verb say, the Subject postposing is quite normal. However, fronting of so used as a manner adjunct gives quite a literary effect: So endeth the lesson, So began my first term and so forth.
So is not a conjunction or conventional discourse connective in either of the Original Poster's sentences - although the word so can be used as a connective as is illustrated below:
- He took my ice-cream, so I punched him on the nose.