Not Permitted. You must retain access to that LB when installation is complete, just as if it were a junction box.
Invariably, someone at this point will say "but we can remove the stones later." They might even find code language that would allow, say, minor amounts of leaf mulch to conceal outdoor lighting fittings. Nice try; we might as well just eliminate any restrictions on accessibility if we're going to stretch that loophole that far.
You'd be better using another code provision that might let you re-examine 'conduit fill,' and make a sweep using a conduit barely large enough for the cable to enter. Or, for that matter, use a cable that's rated for direct burial.
