I have never done this myself (I want the Frequent Traveller points) but know people who have.
It usually works, but every so often they will do a random check of passenger names just before you reach the check-in machines at St Pancras or the Gare du Nord (don't know about Brussels). If you are caught, they will just take the ticket and turn you away (they are quite accustomed to people doing this, which is one of the reasons they make random checks). Generally, I would give people an 80% to 90% chance of success with this method (but, again, I don't do it myself, and I'm not advocating it here).