I dont really have anything of use to add so please excuse my post, but have just spent a little while reading about channel bonding and found it interesting.
First page I ended up on was
here, with a chap that has rolled his own bonding without help of an ISP by using VPN tunnels to a VM he rents in a datacenter, sounds complicated but pretty interesting, from examination of AAISPs wiki they seem to offer hosting to facilitate this type of link aggregation, but I guess the overheads would be far greater than your current solution and I have no idea if the upstream load balancing would be any better.
I then read a little about MLPPP (multi link ppp), the RFC is from the late 90s (not always a bad thing) but it seems its use didnt get much further than ISDN lines. Did find an interesting project
here but it appears to have been abandoned some 8 years ago. Again, does it load balance any better?
So if I understand correctly AAISPs clever routers do some magic to determine which link to send you data based on constant monitoring of latency on each line, but egress distribution is down to the firebrick on your end. I have little doubt this close to state of the art, do AAISP themselves not perform the downstream task using firebrick type equipment? I would hope its more sensible than just round robin with proportions determined by stated link capacity. You have to tell the firebrick each lines current syncs though right?
It would be fun to try out a linux based router to see if some magic combination of pings, packet monitoring and telnet/http/etc interrogation of the zyxels could achieve a better distribution of egress packets, but probably requires a wizard.
One last thought about something you mentioned in your forth line thread, if automating the process of querying the modems and pushing a config to the firebrick remains difficult with ipad tools, a handy dandy raspberry pi could probably simplify the task. Could sit there on all the time consuming a few watts, polling each modem and updating the firebrick as needed.
Anyway, sorry for the ramble!