Nasim, thank you so much for the response. It's definitely a strange move by Nikon; I make in-camera multiple exposures often enough to really appreciate the flexibility of the RAW file. Multiple exposure file output isn't something that's discussed often in camera reviews, probably because it's a fairly niche feature and a lot of people either prefer to blend images in post or are content with a JPEG. Based on my own research checking through the user manuals available online, it seems like most mirrorless cameras - Nikon, Fuji, Panasonic/Lumix, Ricoh - that offer multiple exposure modes all have made a trend out of limiting the in-camera output to a JPEG across all models.
Canon seems like the one possible exception here. I haven't read through the R6 manual yet, but the R5 manual suggests that multiple exposure output corresponds to the set image quality. I'm assuming this means if RAW output is available when making a multiple exposure, a RAW file will indeed be saved.
I doubt it's a high priority on Nikon's list, or if it's even possible, but a firmware update giving this capability to existing Z cameras would definitely be appreciated.