I know that this is probably long dead, and my posting in it three days after the last person not be the best, but:
There was a thread in the Barrels & Paint forum about how to make a working silencer: http://www.pbnation.com/showthread.php?t=2442725
The biggest things brought up on the legalities of owning a silencer for a paintball marker where:
1) It must be permanently affixed to the marker. If the barrel can be removed from the marker, then it becomes illegal. BUT, see #2 and #3...
2) The BATF did take a marker with a permanently attached barrel, and did hacksaw the barrel off the marker. They did this for a test, and did not try to imply that because it might have been easy to hacksaw off the barrel, therefore it was illegal. But they did find that keeping the silencer on the barrel, and shooting a Ruger Mk. II .22 caliber pistol through it, lowered the sound signature by 8 db.
3) If you want to pay the $200 tax, wait the appropriate amount of time, for a background check, and you live in one of the states that allow you to also own and use a silencer, then you have no problems. But only if you the silencer can be removed from the marker itself. If the barrel is permanently attached to the marker, the body for instance, then it can be legal, without having to spend the $200 on the tax, or going through the background check, as long as you still live within one of the states that allow you to own a silencer.
Truth be told, with the Apex barrel out there, and the looks of the Tippmann Flatline barrels, you could probably make yourself a silencer and as long as you did not tell anyone that you had a silencer, you might not have any problems. And most likely, you won't ever have any problems with the BATF. But that is the chance that you take.