This is something I probably should have noticed during the development of BBCM, however, I didn't. See the following thread for how this discussion came about: http://xenforo.com/community/threads/modify-img-bbcode.24264. His attempt is to build a proxy for the img tag so that mixed content warnings on a fully https system. His proxy (thanks to some guidance from xfrocks and I) is working on posted threads. However, on every sub-class of the system it directly extends XenForo_BbCode_Formatter_Base directly. This skips the possibility for the system to allow modifications of the img code.
Although, I noticed in WYSIWYG the class, the img BBCode isn't even defined. The system inherits the possibility to display the system, so in theory, shouldn't it also inherit non-standard codes? My proposed solution was to extend both XenForo_BbCode_Formatter_Base and XenForo_BbCode_Formatter_Wysiwyg within his listener to gain the desired effect.