In theory, it may be possible but in practise it may be more dangerous for the Dark Gods than other methods.
While it is possible for the various gods to "hide their actions/plans", IIRC the Gods of Light are apparently better at it than the Dark Gods. So it is possible that when a Dark God attempts such a stunt, one of the Gods of Light interfers in the attempt risking a fight that neither Dark or Light want to occur due to the danger to that world.
It may be safer (and harder to stop) for the Dark Gods to place their servants into positions to cause trouble.
Consider the trouble caused by the servants of Shigū when they posed as servants of Lillinara.
Of course, in cases like Cassan, there would be no need to fake the appearance of one of the Gods of Light.
Cassan didn't need encouragement from one of the Gods.
ericth wrote:When the various gods make appearances, like say Liliarna to Leanna and Gahyrfressa, it always occurs to me if it would be possible or practical for a member of the other side to take on the guise of an opposing god and visit a mortal.
Think back to when Bahzell was not being first recruited. A skillful Dark God might want to "encourage" Bahzell to not be receptive to Tomanak, perhaps by pre-empting the visit of the musical goddess (whose name escapes me) and pulling a Sir Yohrus and making the situation worse with deceitful words.
Or perhaps giving Cassan reason to believe he was actually doing the right thing and the gods approved!