The way I am doing it is by keeping box selection on while using the transform, move, scale, and rotate tools.
No more needing to hit b all the time.
Keymap - 3d view - mesh - box select
Copy text
View3d.select_box
Now open any tool you want. Go into or add a tweak event mapping. Paste the text you copied into there. Keep modifier keys off or change to what you want.
Add new key map item, change event to left click. Change modifier to shift, change mode to add.
Add new kep map item, do the same thing again but use ctrl and subtract.
Obviously substitute what I am using for whatever clicks buttons and modifiers you want to use.
One compromise is you have to drag to add or subtract selections. Hell of a lot nicer than hitting b or whatever you want to map it to, selecting, then going back to the other tool you want to use.