To get some Async loading I think the best would be to write a function in C to load a file in chunks.
However you mention that that you use fileplayer. To my knowedge this type is already loading the file partially, at least it should load the content to fill the buffer. Did you already try to change the buffer size so it would have to load less data?
I also wonder what happends when you call playdate.sound.fileplayer:setBufferSize(seconds) to increase the buffer size. Does it reload the whole buffer or does it load just the bit missing. That would be interesting to try.
For reference for people who might come here in the future, I also put the post Shaun who provided a solution if you have a lot of assets to load.