Do you have a "mixer"? Like a siphon that pulls 1/20th as much as the volume of water flowing through it?
You can attach that between a hose end and any nozzle you have, but you might have to carry the bottle of concentrate in one hand and the hose in another.
Like:
Hozon Siphon Mixer
http://www.hozon.com/
I paid $18 for one.
I think it is designed for high-flow-fertilizer injection, but might work for adding a 20X concentrate of bug-spray.
I don't know of an injector that can be dialed down to, say, 1:1 mixing.