I would say to try out what you have suggested this weekend. If it makes everything smoother then it saves you time and makes it more enjoyable for the players, if it doesn't work and it ruins the whole tournament and everybody gets pissed off, oh well, there is a new tournament in 7 days.
I know for a fact that it makes everything smoother because the system I talk in the original post is exactly what I used last Saturday. I simply took all the names in chat and built the brackets. Whole process took less than 2 minutes. From an organizers perspective, it was awesome.
I see 2 possible fail scenarios here:
1. Too many people show up
2. People who show up are not playing
From the feedback I've been hearing, I think next Saturdays tournament will be the most popular tournament ever. This is why I'm a bit worried that a lot of people might show up, causing all kinds of problems during Brackets Building and also during round 1.
#2 is a problem, but if people are not playing, then the opponent simply gets a bye. It shouldn't be that big of a deal really but we'll see.