A technique I use for heat-sensitive locations is to not worry about making the joint the first time around. Pre-tin your "spot" on the rail with a quick, light application of heat and solder. Let cool. Do the same with the wire (use locking pliers, c-clamp or other for holding the wire). Then hold the wire at the precise location you want to solder it to, apply iron tip to wire, and push the wire into location, then remove the iron quickly and let the joint set. The problem with doing it all at once is juggling solder, iron, and wire inevitably takes longer than the above technique, leading to melted ties, etc. It will take a few tries to get the temperature right, high enough to avoid bad joints but not hot enough to melt ties, and this could easily be practised on a scrap piece of flex.
I find with this technique I can reliably do in-situ drops for points, rails, joints, or whatever is needed with few-to-no incidents of damage. I walk around the layout and put in 40-50 solder spots on the rail bottoms at my previously drilled holes using a fine tipped iron, then walk around and add the drops. Usually the only thing that suffers is the cork surface(minor heat charring), and that will be coated with ballast anyway. This is with rosin-core, so my guess is using solid and flux would go even slicker.
If soldering to rail already ballasted, it will be tough to get tip and wire underneath, but a fine stripped wire can still be applied to the bottom edge of the rail without interference issues.
Works for me, anyway.