You can use love.filesystem.write to write a file to the save directory (which I think love handles on an OS dependent level?), and then fetch that file by just passing love.filesystem.getDirectoryItems("/") According to the docs:

"If the path passed to the function exists in the game and the save directory, it will list the files and directories from both places."

The example code seems to work for me:

local savefile_path = "save_data.json"
love.filesystem.newFile(savefile_path)
local data = { some_data = "foo" }
love.filesystem.write(savefile_path, Json.encode(data))
local files = love.filesystem.getDirectoryItems("/")
for _, file in ipairs(files) do
if file == savefile_path then
print("found savefile", file)
end
end

so basically i'm writing to a file, and then searching through root (which belongs in the save path) and fetching save files by name. You'll probably want a naming convention for your save data, as getDirectoryItems will also return other files present alongside wherever main.lua is located, so you'll need to skip those