Look up the MacIIHD20 INIT.
I've heard of this, but never seen it. Do you have a link where it can be found?
An init would be nice, but still not as good as having support in ROM, because you couldn't actually boot from the HD20 if you need to load an init from another disk first.
I would certainly believe that special disk transfer software could also allow for HD20 access from an SE/30. There's no physical incompatibility, after all, it's just lack of a suitable disk driver in ROM.
A replacement ROM would give the SE/30 built-in HD20 support, as well as make it natively 32-bit clean. And if you get the dougg3 rom simm, you can also get other goodies like a built-in bootable rom disk, custom startup chime, custom happy mac icon, skip the ram test, etc.