It's a viable solution to the problem at hand. Personally I think using a counter, table or other hard object for slide manipulation is much safer.
Another option beside placing the weapon between your knees, which requires standing, is to squat or kneel behind cover and and place the pistol behind your knee capturing it in the crook there to insert the mag. Then use the heel of your boot to manipulate the slide.
Personally I'm a firm believer in if you've ran your CCW dry you've done screwed up anyway. In most situations such as robbery, assault etc the threat will be neutralized long before the bottom of the mag. This is due to the close nature in which these situations present themselves in the first place. In a situation such as an active shooter you should immediately seek cover in a defendable position. A person intent on doing mass harm is not going to spend time trying to root you out of that position while being shot at. Their goal is mass casualties of defenseless people. Logically as soon as he's met with resistance from a defended position, he's going to move on.
In reality this mostly only applies to situations where you're on the offensive. As a CCW holder it's up to you if you want to go playing Mr hero I'm going to neutralize the threat and go on the attack. It opens you up to additional legal ramifications of hitting unintentional targets as well as the increased odds of losing the battle. As for me and mine my weapon is for my safety , while it sucks to be those poor defenseless targets, living life unprepared and unarmed is a decision they made, it's not on me to bail them out when the shit hits the fan.
