Processing Inverted English Script
Are there any good experiments on the phenomenon of processing inverted text?


This is probably the place to start:

Poldrack, Russell A., et al. "The neural basis of visual skill learning: an fMRI study of mirror reading." Cerebral Cortex 8.1 (1998): 1-10.
APA 


  What are the underlying mechanisms hypothesized by the quoted papers in processing such text?


"Multiple regions in the occipital
lobe, inferior temporal cortex, superior parietal cortex and
cerebellum were involved in the reading of mirror-reversed
compared to normally oriented text. For novel stimuli, skilled
mirror-reading was associated with decreased activation in the right
superior parietal cortex and posterior occipital regions and
increased activation in the left inferior temporal lobe. These results
suggest that learning to read mirror-reversed text involves a
progression from visuospatial transformation to direct recognition of
transformed letters. Reading practiced, relative to unpracticed,
stimuli was associated with decreased activation in occipital visual
cortices, inferior temporal cortex and superior parietal cortex and
increased activation in occipito-parietal and lateral temporal
regions. By examining skill learning and item-specific repetition
priming in the same task, this study demonstrates that both of these
forms of learning exhibit shifts in the set of neural structures that
contribute to performance."


  Does there seem to be any evidence of rule-based processing?


Yes:

"Mirror-reading skill is letter-specific rather than being a
general visual transformation skill"