I've not done that kind of SW in a long time, and never with someone else's platform. But that said, the reporting application I was describing above was a platform in the same sense. It was largely an engine for generating the forms being filled out by the end users. Which is why it there was so much effort doing usability stuff, because the underlying form descriptions had to have tons of optimization flags for doing things like list sorting common items to the top N items of drop-downs, or moving fields around in the form to match the ways the users thought about filling out the forms.
So there were two sides, the engine optimizations to assure things like tab orders on a form, and the was the actually writing the form descriptions. In the first couple organizations that adopted it I wrote the forms and the engine in parallel adding feature flags/controls as needed to support the desired UI outcomes. Later after I quit, the lady who wrote much of the RFP responses started writing the actual form descriptions because it was just as easy as drawing them out in the (visio?) plugin she was using with MS word and doing screen captures for the RFP. Then I guess because she knew how to do it, was doing the "tuning" as well.