Best practice is to always boot from MSFC bootflash and not slot0:. However if you want to and provided you have the bootloader in the MSFC2 bootflash, you can boot from PCMCIA..Here is an example
rommon 1 > boot sup-slot0:c6msfc2-jsv-mz.121-11b.E4
Self decompressing the image : ######################################################################################################################################