I have really simple needs here -- I just want to (effectively) replace the
entire partitioning phase of installation with a single shell script -- I
need to do all sorts of interesting things with partitioning, but I can
easily write it out.
Well, I dont understant precisely what you are trying to do but see if this helps.
If you are looking to automatize the partitioning phase you could preseed that part of installation[1] leaving the things more simple and silencing the installer. If you are looking to remove that installation phase you could delete the d-i modules (udebs) responsible for the partitioning stage from cdrom/local repository/whatever.
Are you looking to replace that stage for another or just remove it?
Is there a quick-n-easy way (or already-documented method that Google isn't
telling me about) to do this? I'm just hoping that somebody's already done
this, and can either tell me how it's done, give me some hints, or tell me
that it can't be done. <grin>
Both methods are quite simple. The simplest way is preseeding the entire thing but how I dont understand well what are you trying to do, I dont know if this will help.