Not your fault, as I've looked at the code, but it's a shame I guess that when it's run there is not a lot of context information around to pull from.
I'd hoped to be able to have access to the target destination field definition already,
(My use case is trying to rewrite an imported string into an entityreference reference ID, and if I knew the target field definition info I'd be able to do the lookup better)
but I guess I have to do all those api lookups by hand. :-(
I was able to push the module here to give me the 'source' and the 'settings' arrays ... but not the target info.
No action needed here, just a little comment from me.