If I am wrong, there are at least two widgets, with open code, which display the favicon set by MicroB. At the least, their code could be checked to see if they do anything special or, if not, just display the icon - set by the program - as-is. If it does just display the icon as-is, then it's more in favour of browser compositing the icon.
Oh, just to check that it wasn't done in the engine, I built the extremely old tablet-browser-ui-with-source (*gasp*)
P.S: Nice work on that browser.