The best practice would certainly be to use bolts and standoffs with the mount holes on Tessel and the RFID module (did you mean module or card?), but that would make rapid swapping difficult.
Velcro is probably not a terrible option (I wouldn't be too concerned about static). Regardless of what you choose, I would place the mount solution in the center of the RFID module, rather than on the Tessel.
Depending on the finality/polish required, you might also look into using something like sugru to make a custom compliant holder or, even more lo-fi, a bunch of those brass fasteners stuck to the underside of the table (just avoid their touching exposed metal).
It's also worth noting that the RFID module can read cards that are "below" it just as easily as ones "above" it, so you can mount the assembly upside down no problem, which should make the mounting problem a little easier.
Sadly, quick Googling tells me that I did not size the mount holes to mate snugly with Lego bricks (the holes are a tad bigger than the bumps). sigh... Something else for rev 2.