Can anyone suggest strategies for coping with a 200MB/day Internet quota?

I hope you remember back to the days when 200MB/day was an incredible amount to have. :-)

There are some simple things and then some more complicated things.

Limit access by time of day

First of all, you can block times of day for connections by MAC address (advanced settings) on the Airport Extreme base station. This can keep machines from pulling down files, updates, etc., without your permission.

Cache data with a proxy server

Second, you can install a "caching" proxy server, such as Polipo http://www.pps.jussieu.fr/~jch/software/polipo/, which will allow multiple machines to access the same content, fed from your iMac instead of the Internet. You'll need to perform the same sort of proxy chaining as you currently do with GlimmerBlocker. You should look around for a caching proxy that offers plugins for ad blocking, etc.

Limit your bandwidth so you cannot exceed your daily transfer limit

Third, you could perform what is called "rate limiting", "traffic shaping," or "QoS" (Quality of Service) (see http://en.wikipedia.org/wiki/Traffic_shaping). You can do this by installing software (http://intrarts.com/throttled.html is one I have Googled but have not used) or by purchasing a home router that offers this option and putting it between your AE base station and the satellite router. Here's a list from CNET: http://reviews.cnet.com/routers/?filter=500563_5554972_

With this option, you could do some rough math to figure out, given your peak usage, how much bandwidth to allow to your network. I don't think the software solution will work with your iMac proxy solution since people do not connect through the iMac (but the caching proxy will help) so you may have to spend $50 or so on a router and configure the allowed bandwidth.

This will basically cause you to treat your 200MB/day satellite connection as a 18kbps modem assuming 24 hours a day usage. If you really only use it eight hours a day, you could have the equivalent of a 56kbps modem. Fudge up or down based on your comfort level. It will stink but you won't have overages, and you can always "turn it off" if you need to make a big download.

It will also allow your entire family to download video or other media. It will be self-correcting, in that it will be so painful to download high definition video that it won't be worth doing.

Most of the useful stuff is proxy side, but I'd recommend disabling Flash on every machine if you've not already done it, then disabling images in browser (to do it on Safari you need to enable the Developer settings). Not the best web experience but you'll stay under your bandwidth limit.

I'd also recommend disabling the Apple automatic software updates, they'll try slowly pull down the updates so they're ready to go. If there's a security one you need then download it on one from the Apple website and install it locally.

I don't see why your modem has to be "the" DHCP server. You can simply run another NAT behind the router using your Airport Extreme. In that case the modem will have only one IP address to serve, namely your AE.

To solve this properly you need to control all internet traffic of your household by putting some server between your household and the modem. One way of doing that would be to use an extra USB ethernet dongle in the iMac (or if the cable modem has wifi, a separate wifi network). Then you can intercept all outbound traffic on your iMac and put a transparent proxy in place.

The Squid proxy for example allows you to configure it so it blocks HTTP transfers over a certain size.

It's not easy to set all this up under OS X, but you could run a Linux firewall distro in virtualbox on your iMac.