vc. I commend you on your pursuit of a solution for this...it would be a neat trick and a good way to learn about what linux does at startup.
I am at work so I don't have a version of PCLinuxOS to take a look at, but it seems like generally the dilemma would be that the commands to create the filesystem (mkreiserfs) are located on...you got it, a filesystem...which need to be mounted by fstab.
So fstab first mounts root, and at that point the commands are accessible to create your filesystem. I would assume fstab is special in its ability to be processed before root is mounted. If we could delve into understanding the boot system and how it accomplishes that, then perhaps there would be an opportunity to get the filesystem made earlier than fstab is processed.
Looking at a RHEL5 box, I see /etc/rc.d/rc.sysinit does the mounting of /proc and /sys near the top, then later does all the mounting...some manually, then mounting all local systems with a mount -a. So there would be your opportunity I think to jump in and make your filesystem and mount it. (right after root is mounted).
But if you have this file make your filesystem and mount /var and /tmp someplace in there (try different places), do a comparison of the contents of the /var directory when you do this, and when you don't. (copy or tar off the contents for comparison) This way, you could see if you got it made early enough such that the directories made underneath have any actual data in them, or were still empty when you mounted the ramdisk. If there is no difference, you are golden. Whatever your ramdisk (or tmpfs) version is missing, is being written to the prior mounted directory I suppose, doing what you don't want (writing to flash).
I mentioned tmpfs above because you could try this comparison using tmpfs mounts in fstab as well. If fstab is processed before rc.sysinit, then that might be the preferred method to test. If you try this, let me know how it goes and I'll be happy to keep thinking with you.
Keep up your hunt...will be rewarding when it works.