If Ashrian has stunned an enemy by being next to one at the start of the monster turn, can the OL still play Dash or Frenzy on the stunned monster to have it get another move / attack action?

 sure, why not? xD he can and he should do it 

This is my thinking on why the OL wouldn't be able to:

The stunned status says "As an action, discard this card or token. This is the only action you may perform on your turn while you have this card or token."

Dash and frenzy both start with "Play when activating a monster. That monster may perform an additional move / attack action this turn in addition to its normal 2 actions."

So if the only action it can perform the turn it is stunned is discarding the token, then I would think having an additional attack / move action would be negated by the stun status. I don't think playing either OL card would allow that monster to do anything.

 This should answer your question:

"Q: Does the Stunned condition take away a figure’s entire turn?
A: No, the Stunned condition causes a figure to use its first action to
discard the Condition card. After a Stunned card or token is discarded,
the figure is free to perform a second action. If a figure becomes Stunned
during its turn after performing its first action, its second action must be
used to discard the Stunned card or token."

Its in official FAQ.

Stun only is for the first action, so you still have a second OR, if overlord play dash or frenzy he can move or atack again (or both)

Thanks, that clears it up.