Great point John. Small bikes, in so many ways, are ideally suited for travel down south. But there are always ups and downs to any compromise.
If you like to explore dirt back roads a lot and plan to stay off fast main roads and can actually ride dirt/sand/mud with some skill...then all the better with a smaller/lighter bike. No need for big power. If you don't plan to go off road, then, to me, there is no point in taking a small-ish bike. A 650 would win in every other practical way. (less stressed, easier packing, more comfort)
Starting in Baja, well, you can have a ball there. You could spend a month easily doing off road. I've ridden the original Baja 1000 course for years. Awesome!
Mexico mainland you'll have to search out the dirt but there is PLENTY...like along the whole Sierra Madre, Sierra Taramuharu, and more. And of course lots of little paved roads everywhere that will avoid big highways. But from time to time, you WILL end up on a fast road. Daylight riding only in this case.
In C. America if you go over the Caribe side you'll find a lot of mud, sand and jungle. There aren't many super highways so if your bike is comfortable at
60 mph, then you're good to go IMO. Getting out to remote villages the little bike will have big advantage.
Getting the bike from Canada, through USA to Mex. Best ride off interstates and on small backroads. If you need a route through Nor Cal, I can
map out a very entertaining ride....with dirt included, if you like.
L.A. is tougher but doable. Once in Mex, home free, stay off
$$ Cuota roads $$$.
Luggage: Most 250/400cc bikes may not have the strongest rear subframes.
So packing a lot of weight could present problems. The more forward and low you keep your load, the better on the frame. Using tank panniers will be a must.
Seat comfort: Limited aftermarket seats, stock ones are very narrow.
Vibes: More than a street bike on some smaller bikes. There are solutions.
More frequent Oil changes: On 250's especially.
For me, there is only ONE bike that covers most requirements you seek.
That is the Suzuki DRZ400S. I owned an E model for dirt only riding but have ridden with several guys who own the DRZ-S model and we've done major trips here in the US and Mexico. This is one seriously reliable bike. Since its street legal, its set up to carry a passenger so rear subframe is quite strong.
Though not a featherweight, the DRZ-S is still about the best compromise I know of. It's about 50 lbs. lighter than my DR650, is nearly as fast, and handles dirt and rough stuff MUCH BETTER. And...it can be loaded up nicely
and I'm pretty sure you can get a Corbin (or other) seat for it.
On the highway tha S model is comfortable at 60-65 mph all day, and can go 90 mph if need be.
Honda XR400: Sub frame will crack if overloaded....never designed to carry a passenger like the DRZ400S is.
XR250: (I own one) not a highway bike IMO. Wonderful dirt bike. The XL street version could be good. I feel the Suzuki will last longer without issue.
Yamaha XT250: The street legal version is a good choice, but limited load carrying, modest power for deep sand/steep climbs and sustained highway cruising. But if doing serious exploring, could be really good.
I would pick a bike a take a week of two trip around Canada/USA to check it out. See if it works. See if you like the pace, the comfort, see if you can
really pack it up for a 6 month ride to S.A. carrying all the spares and long ride
crap you have to haul along. Thing about RANGE, comfort, vibes (fatique) and
packing it up.