Yes. The 512K will access the 800K drive as a 400K drive with the 64K ROMs. You can also format it as a bootable 800K MFS drive which is an amazingly useful thing! Follow these instructions, which will also explain the problem with the 800K drive cable. Unless you have a specific 800K drive, which blocks the PWM signal at the header, you will need another cable or mod the existing one.

However, using the HD20 INIT with System 2.1/Finder 5.0 and up will allow the 512K to write natively to 800K HFS drives and basically turn it into a 512Ke for all practical purposes, without the ROM upgrade. However, you MUST always boot from a 400K (or 800K) MFS disk, before you can use 800K HFS disks. The 64K ROMs will NOT recognize HFS disks before the HD20 INIT loads (as an aside, 400K HFS disks will not boot any Mac).