There is an excellent piece by Guthrum (Andrew Withers) on the offence of Misconduct in Public Office that helps answer all my own questions about this most valuable offence. From recent trial accounts, the offence is used mainly by the CPS to prosecute plods who have been caught shagging civilians whilst on duty - something clearly so heinous that only an offence with a theoretical life sentence will do. As Guthrum remarks, the one class of criminal conspicuous by their absence from facing this charge in the dock are the most culpably guilty of all - politicians, their dags, and senior public servants.
It's not, I strongly suspect, that the more gung-ho prosecutors in the CPS wouldn't like to try it - but almost certain that their political masters have prohibited them from doing so. The CPS is, after all, a branch of government, not a branch of an independent judiciary. They do what their Permanent Secretary and SoS - the very people most threatened by the offence - tell them to do. Which is not to even think about prosecuting any but the lowliest and most humble of public servants for misconduct.
If the CPS were both independent of government and democratically supported I'm sure we would see some very senior public servants and politicians in the dock alongside the shagging coppers. As it's an indictable offence, those charged have the right of a jury trial; I think the CPS need to trust an English or Welsh jury (I'm not sure that wilful misconduct is actually an offence in Scotland) to decide guilt or innocence. They might be surprised.