Great! Section 4.3 in the printer manual says the special functions can also be invoked by single special characters without using the control font, IF you are not using an "Extended" font. Is the regular font you are using Extended? (see the list in 4.1) Character 81H corresponds to the control font A, and will open the drawer. 81H=129 decimal or  , which by my O'Reilly HTML reference is not supported, but give it a try anyway. You might also try ƒ which is supported in HTML, and is also defined as a drawer-opening command.
There is more relevant stuff in the documentation that I haven't read in detail, but let's try this now. If it doesn't work, we can probe further.