Enginecovens cannot be taken outside Clan Skryre batallion - thats clear. BUT :
1. Do engincovens count as batallions? If yes, they allow taking artefacts as other batalions right?
2. If they are not considered batallions - all batallion restrictions, bonuses and so on - shoudnt be available.
Taking these two options into account when taking Clan Skryre you should be able to take up to 4 artifacts (one base + clan skryre and at least two enginecovens - which would count as batallions)
Secondly if they dont count as batallions - then you could possible take a unit of stormfiends and make them part of BOTH enginecovens. Why? Because in GH - "can a unit take several slots in a warscroll BATALLION? -> No)