You'd be surprised how effective a MR16 12V 50W halogen spotlight bulb is at locally heating the underside of a PCB. At full power you can easily get up to Pb-free reflow temperatures in a couple of minutes. Its a bit crude and doesn't have temperature control, but a skilled operator will have no problems avoiding board damage.
The setup is relatively simple: Make a 3" tall frame to support a rectangle of drywall board as a heat resistant work table. In the middle, cut a hole to fit a small (5 oz) tomato puree tin, which are typically a good match to the MR16 bulb diameter and under 3" tall. Drill the bottom of the tin to mount a high temperature MR16 bulb socket. Drill or punch the side of the tin at the bottom with a ring of holes for some cooling air flow + to bring out the wires. Drill four small holes just below the top rim to fasten it to the drywall board with panel pins radially outwards into the core of the board. Mount the bulb socket on long bolts each with three nuts, one to fasten the bolt to the socket and the other two to sandwich the tin bottom so the bulb height can be adjusted to flush to the top of the tin. Fasten the wall board to the frame, seal the edges with glue and ideally tape them so it doesn't crumble. Bond and pin the tin into the hole in the wallboard with its top edge flush with the surface using some sort of heat-resistant cement (e.g. for auto exhaust repair). Take care not to handle it roughly till the cement has fully hardened.
As MR16 bulbs usually have dichroic reflectors that allow most of the heat to escape behind the bulb, cover the reflector portion of the bulb with aluminum foil, shiny side in, secured with Kapton tape.
Difference between dichroic and aluminum reflector bulbs - get aluminum reflector bulbs if you can.
Connect the bulbholder to a dimmable 12V halogen 'ballast' mounted in the frame, and mount the dimmer control so it is convenient to operate. 'Black' the face glass of the bulb with a marker pen to reduce the glare when it isn't fully covered by the PCB. Optionally stick aluminum foil over the wallboard, and ground it and the bulb can via a 1 Meg resistor to reduce the ESD risk.
Operation: To avoid excessive thermal expansion stress on larger PCBs, while its coming up to the desired temperature, slide the PCB in an orbiting motion to distribute the heat more evenly at the edges of the area you are spot heating. If you want to monitor the temperature as you preheat the board, stick a square of kapton tape in the area you want to monitor and use an IR thermometer. Keep some spare bulbs handy as you will be running it a lot at low power and may also be overrunning it at max with a board over it keeping the heat in, both of which will shorten bulb life.
Sorry, no photos - its at work and we are in beer-bug lockdown here!