Does the RCC permit Catholic laity to answer exegetical questions (i.e., interpret scripture) on a forum or website? For example, suppose a forum member asked, "What is the meaning of the phrase 'this mystery...which is Christ in you, the hope of glory' in Col. 1:27? (You can replace this with any scripture, the point being that the forum member is asking for the interpretation of scripture, or a phrase therein.)

Is a Catholic layperson (laity) permitted to answer such a question, and therefore interpret scripture? If so, what guidelines must he/she follow when interpreting scripture (i.e., answering a question asking for the interpretation of a scripture, or a phrase therein)?

Primary reason for the question: I get the impression that the interpretation of scripture is reserved for the magisterium. If so, wouldn't that also prohibit or restrict the laity from interpreting scripture on blogs, forums, and websites?
First, I strongly recommend you read this article, which addresses this common concern among young Catholics.  

Addressing your specific question:

Of course we can answer these questions!

Any time anyone is in need of help we are, in fact, called to provide that assistance if we are capable.  This is a fundamental part of Christian charity.  Furthermore, answering these kinds of questions when they are asked by Christians has a special place for us as Catholics, as it is one of the primary means by which we are to evangelize.  Please do not think that by Christians I here refer exclusively to the heretical protestant sects.  Evangelization is not something we do external to the Church but rather something we do within it, incorporating others more fully into the body of Christ, whether they currently profess our faith or not.  Pope Paul VI writes about this with much more detail and accuracy in his encyclical On Evangelization in the Modern World.  This continual evangelization of each other and the unbaptized requires engagement with others in whatever way they now know God, from which one progresses to the truer ways of the Church.  Protestants, by and large, put great stock both in private interpretation (the bad/self contradictory kind) and literalist interpretation of the Bible.  In order to help those unfortunately born into a Protestant environment reach the fullness of the Christian faith it is necessary that a certain level of Bible-centered discourse be engaged in.  In my personal evangelization of Protestants I have found it important to be willing to talk about the Bible with them and to give Biblical support for the errors in their beliefs, in order to be able to move onto more important topics.  You are right to treat such engagement seriously, however, and a set of additional things I have found useful and important responding to Biblical inquiries (almost exclusively fielded by Protestants, so if your mission field is different these may be of less use to you) based on my experience as a catechist and evangelist within the Catholic church follows:


Always remember that you are not officially the final authority, and can only share your expert opinion on such topics.  Make sure the people you are communicating with understand that you are not a priest, perhaps opening your post with a little bit about yourself so people know what the basis for your authority is.
Always remember that you are for all intents and purposes the final authority as God has chosen you to receive this specific opportunity and it is quite possible the person with whom you speak will receive no further information on the subject, albeit through their own fault.  Consider not only the veracity but also the relative impact of everything you say.
Matt 7:4-5
Neh 8:13-17
Tit 2:7-8
Jam 3:1-12
1 Pet 4:10-11
Saint Patrick's Breastplate
Be sure to discuss your teaching frequently with God in prayer.
The intercession of the Saints is of great assistance to all Catholics in all walks of life.  Discuss your teaching with your patron saint as well.  St. Isadore of Seville is the patron saint of the internet, btw.
If you start to make a ministry of it, be sure to talk with your priest.  If you know a good priest it's probably a good idea to talk these kinds of things over with him in most any circumstance.
Seek ye not to destroy or offend without cause, but rather to reform, create, raise, restore, bless, edify, love, give, trust, and shape-- and these too not just for their own sake but for the greater glory of God.
When engaging in Bible-centered discourse, supplement your statements with non-Bible-centered support when possible and appropriate.


Again, these are things I have found useful in forming myself to be ready to engage in Bible-centered discourse with Protestants both online and elsewhere, not specifically things the Church says you have to know/do.  

The most important things to take from this are 1) Laypeople are in no way to be discouraged from reading and interpreting the Bible, nor in discussing the Bible or interpretations thereof with each other and/or outsiders, nor even in teaching on or explaining the meaning of Biblical passages. 2) Teaching always confers great responsibility.  Teaching theology confers even more responsibility.  Responding to these sorts of questions should not be done lightly.