Hello - this question is just sheer curiosity (I have no plans) - if someone wanted to attach a bunch of serial ports to Mikrotik for a dial-up modem pool, is x86 the only option? Or is there a way to use some equipment and connect to a CCR or an 1100/etc?

You can use USB to serial adapters. There are drivers for them on all platforms. Not sure about stability, but just get various modules from different vendors and try them out. Otherwise USB 2.0 should have enough bandwidth for quite a lot of modems...

I've tried to connect multiple USB to serial adaptors via a USB hub and it works.

Great suggestions - thanks! For the fun of it, I tried attaching a USRobotics modem to an 1100AH router last night with a PPP-client (and tried server) couldn't get it to work. Tried straight and Null RS-232 cable/etc. Made sure the console was unattached from serial0, set up the modem init string/etc. I'll try again this weekend. I am after a single channel dial-up port as a 'last resort' connection to the OOB network.