Go for it! If you get it too hot, the silver braze will be ruined before the steel will. (I don't know if that's any consolation!) Even heat-treated bike steel can handle braze-ons.

If possible, practice on some similar gauge junk before trying it on a bike you want to ride. Depending on your flame size, you may be shocked at how quickly it comes up to temperature.

Use the right paste flux* for your filler, then let the flux be your guide to the right temperature. When it melts to a clear viscous liquid, that's getting hot enough to where you can apply the filler. Don't apply the filler until the steel is hot enough to wick the silver in by capillary action. But a little too hot and you "use up" the flux. When the flux turns black it's no longer working and you must stop, clean it all off and try again.

* Your LWS (local welding shop) might have decent flux for silver, like Harris, or you can order Gasflux brand. I'm familiar with their Type G ("the white stuff"), never tried Type H ("black") but I hear it's better for higher temperature and/or longer duration, harder to burn it up? Probably someone here can advise you on that. For braze-ons, the heat cycle should be very short so white flux is more than enough.

Here's a short video by Brian Champan showing what to shoot for. Don't be discouraged if yours doesn't go this well on your first try!
https:// flic.kr/p/2a8ZaSP
EDIT: this forum won't let me post a link to a Flickr video, it tries to replace the link with a picture, but it's a video. Apparently not allowed.
So I detached the https:// from the front, delete the extra space to go there if interested