I like it!
I think I'm ok with permanent chains.
Radius up to 2 ft per level?
I don't think we've really talked about summon coven for this much. But something that allows an increase of effective CL in order to bind really nasty critters is what I'm thinking.
One of your proposed chain durations plus the permanency option is fine by me.
5 ft/2 levels is a good radius, too.
If we're keeping the chains immune to damage (other than Str checks),
I think we should just have summon coven increase the CL for things like duration, escape DCs, etc.
Let's go 1 week/level for chain duration.
Based on spell level, 3000XP seems about right for permanency, though we could discount that a bit since it's just the chains and not the whole spell.
Well, we don't exactly know how summon coven will work yet. Is it going to allow some kind of group/cooperative casting like we've done before?
Blech. I vote we leave the "summon coven text" of Binding Cage until we've dealt with Summon Coven. Maybe it will just be a standard effect.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.