Ceri@tone does have a post for a simple non-buffered series FX loop on their mods page. It'll only work on one channel at a time in a 2-channel 18w though. Getting it to work on both channels is a much more intricate endevor (there's a thread here about that in fact).
You could always pop in two independent loops, or if you add a channel switcher you could rig a relay to also switch the loop between channels. Could get ugly in there!
I think either mojo or Metroamp sells pre-built switching networks.
BBQ's buffered loop kit will require an extra tube so make sure your PT can handle the extra heater current if you go that route. Probably won't be an issue but you never know. Not everyone overspecs everything like our Trinity pals!
BTW, BBQ also has a mod kit for adding a gain stage (or two).
For the cascading thing, check out this thread...
viewtopic.php?t=1798