Have to ask about the cost vs return, though. How many mods will never be able to work with OpenMW as with vanilla, and how many of those will require changes for an OpenMW mod that will not work with vanilla? And how big of a problem will it be with mods adding a check for OpenMW to deal with an issue that latter on gets fixed? For example, a mod doesn't work in OpenMW, and a well-intentioned modder (to avoid useless error reports) adds a check for OpenMW that pops up a message that says the mod doesn't work in OpenMW and prevents continuing on; but later on OpenMW fixes the bug so the mod would work, while that check is still there in the mod and prevents using it anyway.
That's one important thing to keep in mind for developing a check; you want to check the thing you're actually interested in, not something that you would assume has (or doesn't have) that thing. Sooner or later, that assumption can change and the check will no longer do what it was intended to.