When Mageseeker is on my battlefield, can enemy LeBlanc still play a Reflection token from Holding?

No, the enemy LeBlanc player cannot play a Reflection token to a battlefield while Mageseeker Warden is on your battlefield.

Here's why: Mageseeker Warden's passive ability says "opponents can only play units to their base." A Reflection token is a unit token (Rule 182.1.d), and the Deceiver legend's ability instructs you to "play a ready Reflection unit token there" — meaning the battlefield where you conquered or held. Because the Mageseeker restriction prevents opponents from playing units to any battlefield, that instruction can't be followed. Per the "do all you can" principle, the impossible instruction is ignored — the token is never created. The Deceiver's ability still goes on the chain and its cost is paid, but the play instruction fails and the copy effect that follows it (which depends on the token existing) also fails.