UDC UK don’t seem to sell the 32", and so by the time I get one shipped from somewhere else I might as well just pay for a 36er one 
I don’t really like the idea of having to order inner tubes from another country, as I use my 36er daily as a commuting machine so need it to have 100% uptime!
You will want to inflate the tube just a little bit before putting the tyre on to be able to get it out of the way for the bead. I have patched a 29 tube in a 36er a few times. The patches look a bit funny when the tube is deflated as they don’t “unstretch” as much as the tube, but I never had one let go. I trust a patched 29er tube over a patched FOSS tube.
I normally just have a patch kit but if you are going on a trek carrying a 29" tube should be just fine. In a situation like that you should practice with all the gear you bring along though just to make sure everything works when you need it and you aren’t carrying any dead weight.
I like the FOSS tube option because it’s light and easy to fit, but I must admit than when I got a flat even when I patched the crap out of it (up to 5 or 6 patches around the hole) it still leaked.
The 29er solution: my only attempts leaded to killing a tube.
I don’t imagine myself trying to fix a flat during a ride with a spare 29er tube.
If one manages to install a 29er tube before the ride I suggest to carry a real 36er tube (FOSS or not) as a spare tube.
The 32er tube could be a good in between solution, but will it last?
Nimbus Oracle 32" tube is a Foss and I don’t like Foss tubes (expensive, no easy to repair, to easy to get, leakages, …)
So on my Oracle 32", I’m using same tube as on my 36" : Michelin Butyl 700 35-47 : 175g (original 32" Foss is 225g)
It stretches easily to 36"?
The weight gain is impressive, even over a Foss!
Yes, Michelin tube is fine and gets longer when you inflate it.
I’m using talc to help the fitting on a Nimbus Nigthrider2 rim (42mm) which is much easier to fit such tube thanks to the deaper profile
I’ve been running a Foss tube for a couple of months and it seems fine. I don’t know why people seem to have problems with them. I’m usually aiming for 410kPa pressure (~60PSI).
I have one on my 20", one on my 29" and one on my 36". I run the 36" at 3bar (roughly 45psi) and I have to put air in it every couple of weeks. The 20" runs at 50 and it needs air every week. I don’t use the 29" much these days, and each time, it’s flat.
My 24" runs on a regular tube. I never put air in there, unless I decide to. I check it regularly, and it never looses anything. Ok, it runs lower, at 20psi.
I don’t think people have a problem putting air/lots of air in their Foss tubes, it’s just that it doesn’t stay inside for very long.
And the stretched tubes don’t loose air? My 36" Foss seems to loose about 50kPa (~7PSI) per week.
When stretching a 29er tube to 36" i think it’s essential to use enough (really much!) talcum powder to reduce friction and wear between tire and tube. This may prevent many flats.
When using cheap tubes that don’t hold air well, you can add a little tire sealant (notubes!) into the tube which seals the micro leaks. That would still be lighter than running a bigger tube.
I sue teflon powder which is much better than talc and sticks better to both rim and tube.
Here’s the thing. They’re great when they work! I had one in my 36er for over a year without a single flat. Yes I had to re-inflate once a week but so what?
When it finally did flat, I tried patching it and got nowhere. I went ahead and replaced it because it had turned a pretty gross shade of green anyway.
The new one lasted about a week - it exploded in my tyre when I left it out in the heat. The hole wasn’t that big so I tried patching it, which again got me absolutely nowhere.
So I’ve since just been running a normal boring 36" black rubber tube. It weighs a tonne, but it does its job. I actually have two, just in case.
I do pine for the lightweight feel of a FOSS, so I really do think I’ll try the 29er tube at some point, but right now I’m happy enough with a normal 36er tube. I did my Manchester to Blackpool ride with it in, and it didn’t flat once and I managed to get up all the hills despite the weight. So as always, train your engine harder!