While this is not fully covered by the rules: rope tricks, portable holes, bags of holding etc. are considered alternate (demi) planes for most purposes. That said, if you were to put say, forbiddance around someone in a rope trick, they should not cease to exist (or somesuch) when the rope trick expires. Similarly, forbiddance should not make everything in your handy haversack inaccessable.
When this was discussed before, the best conclusion appeared to be:
1) You can use travelling spells to travel away from a non-dimensional space, but never to one (supported by the 3.5 FAQ)
2) Items and creatures can leave a non-dimensional space through the normal means, even if prevented from planar travel.
3) Items and creatures cannot enter a non-dimensional space while prevented from planar travel.