The only real argument against it is that Be'lal was balefired.

However, we know that the Dark One can protect his followers even from balefire if he knows of the danger ahead of time and is grab their souls before they die.

Be'lal was killed by the relatively weak Moiraine, and the time differential was very small. And considering that Be'lal was in Tear as part of a trap intended to capture the Dragon Reborn at the moment of his public recognition, it is certainly plausible that the Dark One was watching closely.

After that, everything else works out. Mazrim Taim escaped/was kidnapped by darkfriends from the Aes Sedai in the next book. Like the poor man and woman who became Osan'gar and Aran'gar, his body could have been provided for Be'lal.

Lots of little things about Taim appear in a different light if you consider that he might be Be'lal. For instance, his disgust with the Asha'man learning to use swords could be because the men's clumsy attempts are a mockery of the sport he perfected rather than because he really dislikes the weapons.

And as for the oft-quoted "Let the Lord of Chaos rule" line, if Taim really is only a darkfriend, I think that would be the first (and only) time that was spoken by anyone other than a forsaken or the Dark One himself.