Here's how I do it when I want to go portable. You don't get exact pour pressure but you bump the trigger and it gets a quick shot of Co2 when pressure drops. I've used this setup several times.
Compact CO2 Injector. Ball Lock Fitting Compact CO2 Injector. Midwest Supplies
Here's my setup at home.
The white one holds 4 5 gallon soda kegs. I'll have a 3rd tap for Nitrogen at some point but for now it's just 2 co2 taps and 4 co2 supply lines connected to two separate manifolds with 2 regulators so I can force carb and pour at different pressures.
The black one is my "beginner rig" One tap but will hold 2 5 gallon soda kegs. I've played with the idea of adding a manifold and another tap but with my Keezer build I really don't need to. The only reason the 2 keg is in use right now is that my keezer ran low on refrigerant. I'm remedying that today.
When I want to bottle, I have a short piece of hose that I connect to my tap, I turn the co2 pressure down and pour slowly to fill the bottle.