I've never seen a Mac Plus with a Superdrive - don't know if the ROM would allow such an upgrade. Standard Mac Plus equipment is an 800K disk drive. The SE was the first compact mac to be FDHD capable that I know of, and I think the FDHD might have required an upgraded ROM/motherboard over the original stock 800K SE.
I still think a Floppy EMU might be your simplest and most cost-effective option in the long run, but another possibility is to get a pair of zip drives, one for USB and one for SCSI. If you shop around carefully, you might get both for $60 or so. You can use the zips to get files to your Plus and then use the Plus to write disks for the 512K.
ETA: Ugh, wait, I forgot that you can't write to standard HFS on a modern Mac. I use zips a lot, but I have to use an OS 9 bridge mac as part of the process. (As you're discovering, the classic mac hobby now tends to require a ridiculous amount of bridge work.) Seriously, go the Floppy EMU route, you won't be sorry.