I have a legacy service that takes a fixed length request( non XML) and provides response also in fixed length format. I am planning to publish this as a tModel with overviewURL pointing towards a document that describes this service. Is there any issue with this approach?.
I don't see any problems here. Nowhere does it say that the overviewURL should point to a WSDL document (although the vast majority of applications probably point to a WSDL document or an HTML page) so I'd say your approach is just fine.