First, thanks on the 4k posts congrats, I didn't realize I had been sneaking up there.
Next, I actually like Surface Reflection's idea.
I originally came from the SCP forums (Freespace forums) and it was apparent that there were a few mods that stood out. They're the ones that actually got a whole dedicated sub forum just for their project. The same thing will probably happen here. At some point, these things just start popping up. If you have to ask yourself or other people whether a mod is big, then the answer is always 'no'. If a mod is big, you'll know it without needing to ask the question (for many reasons).
Those are few and far between. These are on the scale of Garry's Mod is to Steam, as well as Team Fortress and all those others that became 'acquired' by Valve. In fact, Garry's Mod, as SR stated, is the perfect example of what's being proposed.
The reality is, is that some mods will be so big, and so complex that pretty much anyone who frequents the forums nowadays will have no problem buying (you may think this is a blakent statement, but the odds are that if you care this much about the game that you're following the forums to the extent some of you are, you'll be more than willing to throw money at the creator).
Specific logistics aside on the 'how to make it work', I think the idea is sound. It keeps the IP in Josh's hands, as well as the QA to prevent crap from being branded as 'official'. It also allows Josh to monetize what originally was his creation and acts as a way to 'license' the engine without explicitly doing so (hell, he could even toss in a small disclaimer that states those mods are a type of licensing). And last, it allows the content creators to be recognized and be rewarded for their work (something we all can agree should happen--recognition for a great piece of work is always good).
All the questions about "How do you handle X", or "How do you take into account Y", or "What about DRM?" are pointless questions when an idea comes up. If Josh decides to say "Yeah, I like this idea", I'm sure we can also agree with the fact that he WILL find a way to make it work.
Rest in a spoiler for those that think the post is already long enough and didn't read.
As a corollary to some of the other points brought up in general;
Any argument to the fact of being "But what if someone buys it then distributes it on their own for free?" is not really a valid argument. This is the basis of piracy. Software piracy will happen, and while there will be people that will do this, it is still based on the honor system. Nobody can stop you from doing this with mods. Nobody can stop you from doing this with the actual game. People do it already with things from the Humble Bundle. The only thing stopping you is your own moral compass and ethics.
Crap Getting Through the Quality Assurance Filter
There will always be crap. There will also be very simple mods. It won't take long for someone to do a simple mod that makes it play the Final Fantasy fanfare whenever you complete a mission. Some people would love that, and some people won't. It might even get incredibly popular. Is it considered a mod worth buying? Probably not. Popularity != instant money grab. While getting any mods recognized as official is a bit of a popularity contest, it also comes with reason. The Freespace mod Inferno just sort of emerged, and most mods that would probably be considered for an 'officially approved' will end up the same way. It becomes no contest.
Monetizing Mods in General
Like with the DRM, there is no easy way to prevent people from monetizing mods, even against Josh's request. If a group of people make a huge mod and hide it behind a paywall, there's still that. It becomes a bit of a legal gray area that each person feels strongly about. Of course, downloading such a mod and then distributing it freely yourself also stays in that legal area. It doesn't make it wrong or right either. Let's not turn this into a debate on whether or not charging for mods for someone else's game is legal or not, that will NOT end well in these forums with the way this thread has been going already.
None of this states that's all one way or another. It's not a "All mods must be bought" or "All mods must be given away free". You'll always find a crowd that wants to give their stuff away and a crowd that believes their hard work should be rewarded. Some also believe by forcing all mods to be free to stifle the creation of mods. Would someone have created a mod if they knew they would get money for it eventually vs. knowing they would never see a dime? That's an incredibly subjective question that unfortunately doesn't have much merit since there is no way to answer it. Once again, realistically, we can only speculate, and trying to prove that speculation as fact will only end in arguments.
Closing Thoughts -- No one should make a mod EXPECTING to get paid for it for a game such as this. It seems to go against Josh's type, and seems a bit egotistic to do so. That being said, if someone makes something that becomes a surprise hit, and actually contributes to increasing LT's popularity and numbers sold, I'm all for giving them a piece of the pie; whether it's recognition, monetization, or both.