Being in the Nevernever might leave a stick of iron or steel alone, or they might decide to do something about it. I could see some of the Fae getting annoyed at the presence of the Bane in their demesne and then develop an unhealthy interest in tracking the object back to the source in the normal world.
There is also the little issue of what happens if the pen holder is behind a threshold (inside a church, a Circle, etc) I would imagine that would prevent the sword portion from being able to replace/rejoin the pen. There is also some interesting comments on page #275 of YS about conjuring a sword, which would seem to cover what the original poster wanted behind having a magic "pen".
Just wanted to note regarding sword weight, given the vast differences in blade style I was figuring the "sword" would weigh somewhere between ~4lbs and 15+ lbs, depending on whether or not the wielder wanted something like a dress or fencing foil, up to something like a Scottish claymore. Having done some checking, a Coldsteel sword-cane has a weight without the caneshaft of ~16 oz/1 lb, though that is for a "sword" blade that is only ~24"/2 ft in length, which is really more like a long dagger than a sword. A sample of a dozen Elizabethan era rapiers on display in the White Tower dating from 1590-1616 are generally between 36"/3 ft to 52"/4 ft 4 inches in blade length with an average of 42"/ 3.5 ft and weighing between 2.06 lbs to 3.12 lbs with an average of ~2.5 lbs. The long and short of it being that a fencing blade of some type (foil, epee, rapier or saber) would be a little bit light than the low end I mentioned of ~4 lbs, but still a good deal heavier than a pen should be. And having routinely carried something of about that weight (~16.5 oz) in my pocket, the weight in noticeable in the way the item rides and how it makes clothing hang.