hell just do a straight model switch, thats what everyone else does (frieza suddenly grows,ssj'ns hair just pops up, krillin magically grows hair etc), due to the hl engines constriction there aint much you can do about that. Hmmm your know those white round pulses that come out of goku while hes transforming?? maybe an opaque one of those could be added in (instead of the muchly suggested flash of light), it be really really fast, just long enough to cover that fraction of a second, and all it would look like is a power burst (it could even be yellow for the saiyans), then have them already have turbo on (aura and all) and it would be totally sweet.