I'd be nice to have alpha-enabled line selectors on Rockbox, however I don't think that is possible because it would be too CPU intensive (taking too much time to render the display, thus making the user interface unusable).
I see no reason at all why this wouldn't be possible, in fact, it has been discussed quite some time ago by myself and JdGordon and from what I remember it was a "yeah, sure...when/if I/someone gets around to it" thing and not a "No...it'll never happen" thing.
(the new skin parser was taking preference at that time if I recall)
Btw, the icons use transparency...and most .sbs will contain some form of transparency also.
Saying it won't happen because you *think* it might take to much CPU time seems a little bit misguided (sorry if that seems harsh, I'm trying to word this as best I can...I just really don't think you thought about your answer).
[St.]