Color me surprised! My 2005 Sharp 37" TV supports 15KHz too!
I had a couple of those Commodore RGB - VGA adapters lying around (which weren't even intended for the A500, I believe) and I am seeing my native NTSC Workbench screen just fine!
PAL still not supported, however. Looks like I will still have to spring for one of those SCART to HDMI adapters.