4
This is certainly an interesting idea. I've seen modules that implement their own search function. In these cases, they don't rely on the XOOPS search engine API. Rather, they provide a search form inside their module and generate page results however they'd like themselves. So for a module that you are personally developing, this might be a desirable path for you to take.
If, however, you are dealing with other people's modules. Well, you are left with hacking them (like christian's excellent suggestion). Do an elegant enough hack and submit it back to the module developer, however, and it just might become part of that module in the future.
But that still isn't really what you're going for (as far as I can tell). You're asking for the type of change that challenges the very design of the XOOPS search API. It would be interesting, however, to add the concept of 'categories' to the XOOPS search engine concept. Each module would have to define in the xoops_version.php file a way for XOOPS to know what categories are available and how to access these categories. The search engine could then be altered to allow for filtering not only by module, but by category as well.
All of this, obviously, would need to be backward compatible with the existing search engine so that older modules can still be useful in Xoops.
Hmmm..... This might be a fun little project to work on. Anyone else intested in something like this??