It still requires modifying the actual epub file, but see
ePub custom Fonts on 700, 505, 300, 600. A similar approach works on Hanlin V3s and V5s, but not with mobile ADE on other EInk devices.
If you want to change the default for all epubs, this is currently only possible using the Windows Sony ebook Library Reader (see
User Stylesheet for Adobe Digital Editions). The same capability is presumably possible on any EInk reader using ADE, but none has made such a stylesheet user accessible.