A quick addition - tack on 'walking and f-tilting' as an approach. Said approach can be mixed up by crawldashing back just outside of Snake's U-tilt range and fireballing (rinse, repeat) or f-tilting (good idea due to its clank-iness? It probably clanks with the U-tilt's invisible hitbox, but I dunno, I'll check that out later). As for Snake's D-tilt (which has not been discussed at all)... I'm still confused as to how it may or may not affect these approaches. There are just so many ways that an approach could be made and could be countered or fail to be countered...
I don't think that going for low ceiling stages would give Luigi that much of an advantage - Snake's U-tilt has ridiculous range and ridiculous Snakeback (it's so strong that it defies the rather weak definition of knockback), thus making such stage attributes questionable. I mean, potential-kill-wise and percentage-wise, I would place a low ceiling as being more advantageous to Luigi than to Snake - but in case I'm wrong, I think a few other things need to be going for Luigi first...
Due to the relatively easy time Luigis have with spiking Snake and aerial superiority, especially off-stage, I'd be more inclined to place greater emphasis on the stage having large, wide open spaces between ground and blast zone and a cramped central platform or set of cramped central platforms, preferably with uneven terrain than to put emphasis on vertical 'shortness'.
Stages with anti-camp gimmicks are more than welcome :-D
Now... I'm going to rapidly jump through a few stages with little to no analysis. Be prepared for mindlessness:
Hanenbow
Believe it or not, Hanenbow comes to mind as a nice stage for this, but if I'm not mistaken, it's banned...
75m
I'm absolutely serious! Snake may move quickly in the air, but he certainly isn't mobile enough to camp the upper level, and the rest of the level with its variety of spaces and platforms that can be attacked through and jumped through and... and... like. Really. I mean... Just look at it! It's almost screaming, "Advantage, Luigi!" with its huge variety of anti-camper mechanisms. Too bad it's banned <_<
Pictochat
While Pictochat changes often, there's a certain 'safe zone' in the lower left that is devoid of all hazards and changes (save for the influence of that guy who blows [pun intended]). Not particularly advantageous to either character (as in, there's no additive effect) - it doesn't change enough to horribly affect Snake's camping game or to allow Luigi an easy approach. The central platform is too big for positive effects stemming from the transformations to even be noticeable...
Castle Siege
2/3rds of Castle Siege (1st [uneven, cramped] and 3rd [wide empty spaces on the sides, relatively small central stage] forms) matches up okay.
Lylat Cruise
Lylat Cruise is... Is... Well, if it doesn't favor Luigi, it probably favors Snake.
Frigate Orpheon
Frigate Orpheon moves around a bit, has a few platforms... and it has roughly uneven terrain. Sounds like a fair match.
AWESOME IDEA I JUST HAD!
Delfino Isle and Halberd... Why do I group these two together? Because their central platforms spend a relatively large amount of time floating in the middle of nowhere. What's so interesting about floating in the middle of nowhere? Rise-through platforms. What's so awesome about rise through platforms?
T3h Mindgames! What if, let's say, a Snake has just thrown Luigi off of the edge of the platform while in mid flight? Let's say that you sort of 'whiffed' a Side-B recovery and Snake is standing there being a happy edge-guarder. TORNADO YOUR WAY BACK UP INTO HIM AND BE LIKE, "WUT NOW!?" *cough* Okay, so, that was uncalled for, but really, just a little idea... It doesn't make or break the match, just adds one more way of getting into Snake's personal bubble (admittedly, this suffers from the edge-guarding Snake dropping grenades at the edge). Actually... No - this is entirely situational and should probably only be attempted, like, once per match (if that!).
But, really, with regards to Delfino, like DMG said, it'd be an interesting match-up... Halberd's cramped quarters, low in-flight ceiling and wide open air spaces should give Luigi some decent chances at getting face-to-face with Snake and taking him OUTSIDE for the match!
Spear Pillar
... that lower level screams 'camp-mania!' to me. The stage gimmicks, in my opinion, aren't near enough in number or widespread power to counter Snake's campiness on the lower level...
Norfair
I'm leaning towards saying that it's advantage Luigi, due to the variety of platforms. The only place Snake can effectively camp is the bottom platform (everywhere else Luigi can get beneath him and start an aerial combo), and even then, the stage gimmicks should eventually force him away from that lone place of solace.
Smashville
*CRACK* Snake just put me to sleep... No anti-camp gimmicks, almost zero stage variability. That moving platform doesn't do jack to assist Luigi.
Rainbow Cruise
It's already been accepted that RC gives Snake the shivers. Yay for the green thunder!
Okay, that's all that I feel like doing... I'm sleepy... And my abilities of rational thought are failing me. I may have made a few logical errors within the above post - feel free to bash me for them :-P