The MK5,in conjunction with a NEO-2/3 of some kind will allow you to access storage cards.
The MK5 itself has no slot for external memory,but they are pretty big(2-8G,I think:16 to 64g)in the first place.
Also,I'm not sure about"FPGA"(no clue what you are talking about,really..hehe),but if it has to do with the source code,NEO has been known not to release those kinds of things.
This is not FATlib,or even Moonshell or DSOrganize.Like it or not,it is not a public project,it is a business.
NEO has taken some steps recently to improve homebrew support,as have the coders of the homebrew itself(DIDL or somesuch).
Keep in mind,for all the"free"homebrew games to work,there must be commercial equipment(i.e.-for profit)available to run it(or emulators,I suppose,but that does not seem the same).
I kind-of didn't answer you there on that last,just explaining the justification as to why(I think)some source is available,and some not.