...I mean "assigning a title attribute to make hover-messages appear"...
Unfortunately, that is not possible with the current CG system. If you are really good, you might be able to do it with Javascript but I don't know how to use js. It'd be dead easy with php but as you already noted, CG does not support php.
It
is possible to force it by doing your updates by hand (and not use the AutoKeen system at all). This would defeat half the purpose of being with CG as you would be doing each update by manually writing out the code, but it would give you more control over the look and feel of the comic.
If you can live without this bit, then CG should work out fine for you. It's a great place to get started. However, if it is a dealbreaker (I have seen some comics where there where no speech bubbles - the text could only be seen by hovering the cursor over each character) then you might need to use a paid hosting site. This would give you more control, but you'd need to find automation software that let you do what you were after (and pay for hosting + bandwidth).
Also, if you want to use CG you
will need to change your username on the forum to start with a capital.