Most good ideas have been thought of before. Taking Traveller and altering it for fantasy settings is one of them. Most of the fan-made versions, however, appear to be written as conversions for Classic Traveller rather than Mongoose. The first of these which I encountered was Adventurer by the good Doctor Grognard, followed by The Fantasy Traveller at Alegis Downport, and most recently Wanderer. I've read Adventurer, and it generally seems pretty sensible, but it is a conversion of OD&D / 1e to Classic Traveller, so it has a lot of crufty bits from both systems. Also a nice-looking but relatively heavyweight magic system with magic points, much as with MgT's diminishing Psi score. Doc mentioned the other day that he's doing an update, so I might have to read it again as he goes. Alegis' Fantasy Traveller is remarkably lightweight, but also not quite what I'm aiming for, and Wanderer seems to actually be just kind of a notion on some forums without a condensed rules document. An interesting variant on this category of fanbrew systems is Mercator by Mithras over on the CotI boards, which rather than being a fantasy adaptation is a conversion of Classic Traveller to a Roman Empire setting.
There is one fan-conversion for Mongoose Traveller, Adventurer by Nathan at Platonic Solid. However, development appears to have stalled with only a skills list and career tables for Barbarian, Ranger, and Fighter completed. It still provokes some discussion on the MgT forums, though.
There are two commercial ventures into fantasy for MgT that I'm aware of. The first is Flynn's Guide to Magic in Traveller. Flynn's admitted purpose is to get someone to develop a full fantasy Traveller game using his magic system. However, from the advertising copy of "150+ sample spells", it sounds a bit heavier than what I'm aiming for...
The second commercial product of this type is Netherell, by Terra-Sol Games. It gets good reviews, but is fairly pricy and has thematic trouble as a result of Mongoose's licensing rules, where if you publish MgT-based things, you have to fit them into a subsector (Reft or Twilight; I'm not exactly clear on it). Since I don't intend to publish for profit, I can probably get away with something similar to what Platonic Solid did with respect to licensing (namely not worrying about it), but it is inconvenient to have to mix sci-fi into one's fantasy.
Coming next week: a Traveller magic system of my own devising.