It's kind of tough to put markings on a grass strip. You couldn't paint the markings on, they wouldn't last. About the only two ways would be to dig out the grass, down to the dirt, to form your numbers, or, at least in the summer, keep a goodly supply of Roundup on hand
As for the the FAA's relationship with small/private/grass strips, while they keep some sort of file on them, they aren't as concerned with these strips as they are about larger, public use facilities.
Somewhere I've got more info, I'll root around for it.
Tom at MSY
"The criminal ineptitude makes you furious"-Bruce Springsteen, after seeing firsthand the damage from Hurricane Katrina