Just a bunch of Euro's messing with my district while my corp is at work/school. Do not want. Couple changes could make the idea workable.

-Prescheduled
-No alarm system, or at least participation by the defenders must not be contingent on some alarm system.
-relatively minor damage inflicted on the defender, i.e. you shouldn't lose your district or take "significant" clone loss for not showing up for one of these.
-starting such a mission doesn't lock the district, starting such a mission requires some small isk investment (5 million?)
-squad vs squad only (maybe allow additional defenders).
-limited clone count, 25 or lower, perhaps allow more for defender so attackers have to go for objectives
-counterstrike-esque objectives. multiple targets, hack one, if not back-hacked in time attackers win.
-short matches, 20-30 minutes max.

I'm not presenting a complete idea here the way you were, I'm just saying I think these points must be met to have something that's reasonable and workable.

The other question is, what need is this filling? For one, I think that attacks like this would not compromise districts significantly, but if a corp with lots of districts had these little attacks happening everywhere it would be too much to defend all at once if the corp is mostly a small number of elite players. The result would be death by a thousand cuts as these attacks sap the profitability of owning a large number of districts. This would also provide content and direct revenue towards corps that don't or aren't interested in holding a district. It also provides content in that in a major war, one corp could pay others to carry out these little attacks in preparation for an attack or to merely weaken planets not on front lines. Lets say corp A has a solid squad of six, but not the means or the wherewithal to carry out a full attack, and corp B has to send out the bench warmers to defend, this provides content to both the small corps and the bench warmers.