This details what I've ended up with, after having to install a large number of machines running on a pair of ide disks needing software raid. To speed up the install of these machines, I've created a new floppy image. However, in doing this, scsi support (among other things) have been removed from this floppy.
In src.tgz we have the following files: 
    src
    src/sys
    src/sys/arch
    src/sys/arch/i386
    src/sys/arch/i386/conf
    src/sys/arch/i386/conf/RAIDDISK
    src/sys/arch/i386/conf/RAID-INST
    src/sys/arch/i386/conf/RAID
    src/distrib
    src/distrib/i386
    src/distrib/i386/Makefile
    src/distrib/i386/raiddisk
    src/distrib/i386/raiddisk/raid1.conf
    src/distrib/i386/raiddisk/raid0.conf
    src/distrib/i386/raiddisk/list.local
    src/distrib/i386/raiddisk/Makefile
    src/distrib/miniroot
    src/distrib/miniroot/install.sub

Extract them over top of your /usr/src.
src/distrib/i386/Makefile and src/distrib/miniroot/install.sub are not new files, each file just has a line added to them.
RAID is GENERIC with two changes:
RAID-INST is GENERIC with one change:
RAIDDISK is RAMDISK with a pile of changes.
The install process is simple, boot off floppyR36.fs and go through a proper install. Taking care to setup the "RAID" slices when you do your disklabel. (see step1). At the end of the install, configure the raid devices using raidctl, and initialize them. There is no reason why configuring and initializing the raid devices couldn't wait until step2.
Reboot using floppyR36.fs again, this time (S)helling out when given the choice, reconfigure the raid devices, then run install. This time slice up install to raid0 and raid1. After the install is finished, be sure to set raid0 and raid1 to autoconfigure.
Once Step 2 is completed, on the first boot of the machine, the recovery slice, wd0a must be copied to wd1a. How to do this is shown in the recovery document. Step 1
Step 2
A built floppyR36.fs is available.
A method of recovery is detailed here: recovery.txt
In the install steps, I install site36.tgz, this file contains a RAID and RAID-INST kernel. bsd.raid and bsd.inst respectively.