My client wants to use Adobe Recommendation and integrate the response to be recommended with external Product Catalog/Content System via APIs. My idea is integrate Product Data before Apply Offer Call.
I haven't seen it done exactly has you've described. However, it isn't super uncommon to use other API's for helping get the meta data needed to render the recommendations on pages. I know several clients that just return a list of the ids to recommend from Target and then use that list to call other APIs. In that case they use their own code to render the recommendations. This can be helpful for highly dynamic sites and attributes - like a product price is dependent on various factors.