DC dynamic braking

The motor I need to stop is a 90W Single Phase Permanent Capacitor Induction motor, run straight from 230V AC mains. I'm not able to use a variable speed drive, separate DC injection braking unit or a DC electromagnetic brake. So I was considering DC dynamic braking, by applying DC voltage across the windings once mains power is removed.

For the motor you are describing, your choices are limited. The method you described IS in fact DC injection braking, not Dynamic Braking. I don't know of anyone that makes a DCIB for small 1 phase motors like that anyway, so you would probably make your own. You don't really need to filter the DC, a half wave rectifier works fine for this.

You will likely need a test bench version of your motor so you can experiment with the voltage level that you apply, and you will need to be careful about the protection of that motor. If it has built in thermal switches, as many small 1 phase motors like that do, you should be fine.

You will also need a time delay to allow the motor field to collapse before applying the DC, a few cycles is all you really need on a small motor like that, and you will need to experiment with the braking time as well. If the load on you motor varies a lot, you should consider adding a zero speed switch to detect when the shaft has stopped so you can turn off the brake. When considering the duty cycle of the motor, remember to add the braking cycles as if they are starting cycles, the thermal effects on the motor are virtually the same.