(By the way, Brandon has now confirmed we haven't seen Demandred in TOM)

IMO, Demandred has folded the BT into his larger plans. I'm pretty convinced Demandred has established another well-hidden training centre/headquarter, likely deep in the Blight. It would have been hard to train a full army at the BT, secretely. Taim managed it, with a fairly small faction of 100 men or so. He could not have done the same with hundreds.

I believe Taim will turn out to be one of Demandred's early recruits, which he decided to send to Rand to infiltrate his inner circle, after Rand proclaimed his amnesty. The advantages of doing this are numerous. One of the less obvious ones was that it let Demandred recruit openly and far more widely. Taim's faction's men could openly test men in all areas, without raising attention. Theorically, this could mean Demandred was able to get most of the male DF tested for the ability.

My hunch is that most DF men found by Taim's faction never set foot at the BT. They were rather sent to Demanded. Taim was allowed to keep a few, and to keep the men who were not DF but had shown the sort of personality that could be swayed to the Shadow.

Now that the endgame is near, Demanded is gathering all his forces and preparing for war. The clue I see of this in TOM is Androl's mention that one of Taim's "new men" has gained his strength and skills way too fast. Rand had in fact warned Taim to keep an eye for this, as it would be a sign that recruit wasn't who he pretended to be (ie: he would most likely be a Forsaken in disguise). So, I think that "new recruit" isn't a recruit at all. He's a fully trained soldier/officer from Demandred's army, who was sent as liaison/reinforcement to Taim, in prevision of the endgame.

I don't expect Demandred to risk ever setting foot at the BT before TG. He's about to come, he's waiting for Taim to clean the place of all lightsiders first. I think it will turn out that Taim's men belong to a larger force of dreadlords, with a slightly different hierarchy, ranks etc. The way the BT has been organized is probably ladden with subtle organizational flaws. Taim may have taught specific tactics and strategies and not others, precisely so Demandred would know all the strategies of Rand's men, and how to turn them against them etc.

My theory is that Demandred intends to seize most of the Light's "fortresses" held by Rand in the opening move of TG. He will have targeted cities with Waygates, and favored those with Portal Stones near. I think the attack on Caemlyn is meant to capture the city and population, and to provide a giant fortified coral for the main army of Shadowspawn brought South. Demandred's headquarters will move to the BT, protected by the dreamspike from a surprise attack. I think the full range of the ter'angreal is probably larger than the range at which it is set currently. Demandred might be able to extend its range later, to include part or all of Caemlyn eventually.

If Demandred seize the Stone, Cairhien and Caemlyn, he would have a massive advantage. That's three cities to bring the Shadowspawn armies through the Ways, with at least two accessible Portal Stones nearby.

It's probable Demandred has "surprises" in his army, perhaps channellers from other areas etc. (though people tend to forget there aren't that many sparkers in Shara... men develop their ability late, around 16 to 18 or later... and all male sparkers in Shara are killed at 21... Basically, Demandred would have had to extend massive efforts and resources in male channellers at recruiting, to find the Sharans would can learn to channel (and it would have been difficult, in a closed land, under the scrutiny of the Ayyad. That's where most men can be found, in the pool of those who can learn, not among the sparkers. The BT scheme provided him with the resources to do this in the Westlands and almost openly, and I'm guessing this is where he did most of his recruiting. One just has to look at how many women the rebels managed to find in a short time, in a very small area, in Murandy and Altara essentially. This gives an idea of how big the BT should be, with the heavy efforts of Taim. The notion to channel would not be as attractive to men as to women, so that explains the lesser numbers, but stilll... Taim doesn't appear so succesful at recruiting, when you compare his results over the whole continent with many recruiting teams and for many, many months, with Egwene's AS far less extensive, much shorter efforts. Of course, if Taim's team found many more men than were sent to Rand at the BT... than it all makes sense.