So, if I read your post correctly, you would have one admin page (base.php) and have it load other pages (from a database) based on what the user selects.
Quite frankly, I don't see why that would be a problem, and considering I'm one of those types of people who like to keep as little amount of files on my server, I think it's a pretty good idea.
A suggestion I would make is to have your user table have several "switches" in it, so you can turn on and off different types of pages available to the user. For example, I would create a user table like such:
CREATE TABLE UserTable(
ID INT NOT NULL AUTO_INCREMENT DEFAULT '0' PRIMARY KEY,
UserName VARCHAR(255) NOT NULL,
UserPass VARCHAR(32) NOT NULL,
NewsPage ENUM('Off', 'On') NOT NULL,
PollPage ENUM('Off', 'On') NOT NULL,
FAQPage ENUM('Off', 'On') NOT NULL);
What this would do is store a User Name and User Password for logging in (obviously) and then a list of available pages, in this case "News", "Poll", "FAQ". This would only be available to you, and would allow you to control which types of pages users can have on their site.
Anyway, I think it's a great idea, keep me posted on the progress.