I am sorry if am repeating things you already know.
First you need to understand how booting works. BIOS has list of devices (floppy, CDROM, HDD, ...) it is capable of booting from. It checks for device in given order. If the device is present, it will load first 512 bytes (So called MBR) from that device into RAM and execute it. What happens next is dependent on what was written in that first 512 bytes.
In case linux is used some linux loader (GRUB, LILO) is started and it loads linux kernel into memory hence starting the system.
This is how Plop works. It needs to be installed on any device the given BIOS can boot from. Since it contains driver for standard USB it can even than load and execute first 512 bytes from USB stick hence allowing you to boot from USB. Unlucky for you it does not support PCMCIA USB. So one option for you would be to find any bootloader which support PCMCIA USB cards or write your own one
The second method I had in mind would require building your own kernel or at least initrd. I am not sure haw much you know about kernel modular structure so I will try to briefly explain the problem. Kernel itself is responsible for handling hardware. It uses loadable modules for each hardware. This feature is nice since it allows you to load only modules for hardware you really have thus keeping kernel small and stable. But it may bring chicken and egg problem. For example if you have SCSI harddrive. But module for handling SCSI is stored on that disk so you cannot access it without loading it which you cannot do since you do not have SCSI module loaded yet (hope you understand what I mean).
So there are two possibilities how to solve this mess. First is to build kernel with SCSI support directly build into it and not use loadable module (this solution is really good if you are building kernel for one specific machine but not so good in all other cases). Second option is using initramdisk (initrd). It is small package containing essential modules. It is loaded by boot loader together with kernel, so kernel has essential modules available.
This is actually how does thing in your posted link works. You have initrd and kernel on CD and rest (filesystem) on USB. It will work as good with HDD instead CD. Question is if their kernel or initrd contains support for PCMCIA/USB cards....