No, you're right that files from :read-file-form and :read-file-line perfectly fit as INPUT-FILES for DEFINE-OP. I haven't given any thought on whether it's easy or hard to properly inject them as results of INPUT-FILES.
NB: I don't have much cycles for ASDF at the moment, but I will gladly review patches or fix urgent bugs.
PS: What do you think about releasing ASDF 3.3 after next month, just taking steps to avoid escalating from WARNING to STYLE-WARNING until next release?
—♯ƒ • François-René ÐVB Rideau •Reflection&Cybernethics• http://fare.tunes.org When a pamphlet was published entitled "100 Authors Against Einstein", Einstein retorted "If I were wrong, one would be enough."