In this case, the OED says read is an adjective, meaning precisely:
a. Of a person: experienced, versed, or informed in a subject by reading. Also read up (cf. to read up 2a at read v. Phrasal verbs). Only in predicative use.
This is the same adjective we find in well-read:
1574 J. Whitgift Def. Aunswere to Admon. 754 M. Doctor had beene so well read in the auncient Doctors.
1592 A. Day Eng. Secretorie ii. sig. T3, He ought‥to be well languaged, to be sufficiently red in Histories and Antiquities.
1632 P. Massinger Emperour of East iii. iv. sig. G2v, You are read in story; call to remembrance [etc.]
This usage of read is a bit younger than other uses of the past participle of "to read". The etymology notes for the adjective form say that it is derived from the past participle. The verb form has been around since Old English, while the adjective seems to have appeared some time in the 1500s.
This use of read, they note, is only used with modifying adverbs. So, in widely read, widely is an adverb that is modifying the adjective read. When an adverb modifies an adjective, it can often be in this "inversion". This is, for example, the order of "he is widely known". Another example is "I am really hungry" (and you wouldn't say "I am hungry really" with the same meaning).