Turns out mini hair straighteners go to between 180-200c. Ideal for joining pla.
Have a pair of facing half inch ceramic plates that heat up.

I figure you could either use a rotary tool diamond grinding wheel to cut a suitable sized slot in the ceramic plates - pretty risky.
Or make a metal jig that fitted over the plates and had the grooves in place to put the filament for joining.

And the best bit is that you can get a pair of mini straighteners for under £10.
I have soooo many reels with little bits of filament left. I figure this one is a definitely on the todo list :-)