I've not got any stainless at the moment, but I'll be doing an order later this week if you can wait a little.
Stainless water bottle bosses? yes. PM if you want to get something.
You shouldn't have any trouble using the propane torch, as you're not heating up any large volumes of metal. Only needs 610-670ºC. No problem with stainless small braze-ons as it's 34% silver, there are better rods for structural work.
If you do use those rods (BS1845 Ag11) be very careful, as they contain 21% Cadmium, which can boil off as a yellow flare at only 766ºC, get breathed in or ingested as dust and mess with your kidneys, lungs and nervous system
It flows beautifully for lugged work, but very few framebuilders use it these days, it's notifiable waste, even banned in some countries...It's better for use in controlled temperature/atmosphere oven brazing.
Perhaps look for some 35-40% silver cad-free rods and suitable flux. It's almost as good, but wont kill you. CupAlloys
are good for this sort of thing.
For holding them in place...main thing is to not braze your holding device to the frame as well
... Usual method would be a section of stiffish wire rod (mudguard stay?) clamped to the tube with a g-clamp, with its tip resting on the braze-on. Some people will bend the rod into a hook and hang a spanner or something from the hook instead. It all needs to be held fairly firmly, as there's a lot of force and lubrication when the water in the flux boils off, but not held so firm that the tube or braze-on gets distorted as it heat up and expands.
Here's a really nice pro solution from Sputnik Tool
And a quick bodge
Something in between:
Cheapest (and quickest) option is to flux the braze-on well, use the flux paste to pick up a short offcut of rod on the surface of the braze-on. Hold the piece roughly in place with some needle-nosed pliers and heat. The pliers act as a heatsink to help stop the piece overheating. The flux and rod will melt, remove the heat, holding the braze-on in place as it cools. Probably under 20 seconds a fitting, but you need a good eye to make sure that they're all nicely in line.
All the best,
Pause, take a breath and ask yourself: "What would Rastamouse