The contents of man pages are not groff's business. If a man page must be conditioned by what's on a local system, that can likely be done with .sy. But man(1) should not enable .sy. Enjoy the catch-22.
Another reason to keep the status quo in regard to referring to nonexistent man pages is that some installations provide only man 1. Alas, I use one such. I am very glad to be told that man 7 groff exists even if it's not immediately at hand. I can and do go look for it elsewhere. However, various uses of groff might be helped by environment inquiries, for example, to find whether an optional .so file exists. This could be done by a request like .isfile file string that assigns 1 or 0 to string according as the file exists. Or it could be done by a builtin string invoked this way: \*[isfile file]. (The notion of a builtin string fills in the analogy macro:request :: string:__.). Given the environment query, an adaptive man page would be easy to implement, though unwise. I oppose the idea of a special process for installing groff. That would be a loathsome spawn of autoconfig. I am not excited by the idea of environment queries, either, because (1) their implementation is likely OS-dependent , (2) I am not sure that they provide enough value for cost, and (3) there could be lots of kinds of queries. But at least they would be general-purpose. Doug