Yes, the steel will be glowing at least orange before the brass flows -- with the understanding that ambient light levels will influence your perception of color. If I haven't done forge brazing in a while, I'm always surprised how hot things really need to be.
Just to round out the discussion because others might find it useful, there are some other points to bear in mind when forge brazing. First, is that the better prepared the joint the better the capillary action pulling the brass into the seam. So, try for a really tight and stable joint. Other things being equal. gravity will effect how the brass pools and flows too. It will find a low spot and puddle there. And finally, the brass will also tend to flow to the areas of higher heat.
Confession time: I have never brazed a buttplate, but I have brazed some much more complex items like the eight or ten parts that make up the body of a box iron (for clothing). It I were brazing a buttplate I'd probably heat slowly with the heel down in the coke bed, then as the brass flows (watch for blue flame coming off the brass) I'd gentle tip the piece left then right to give a bit of a gravity assist. My preference is not to leave it in the fire longer than necessary but to take the piece out as soon as I'm sure the brass has flowed as desired. Let it slowly air cool.
If you've used borax as a flux, it can be very tenacious and hard to clean off. It's water soluble so sometimes just a good 24 hour soak in the slack tube will get most off. Or boil it.
If you do forge welding, do not tip molten brass into the fire. The conventional wisdom is that it will greatly inhibit your ability to weld until thoroughly cleaned out.
