Kat,
I've slipped my transmission more than most so I think I can comment here.
Before upgrading my transmission, I could slip my TC clutch at will. Coming home from vacation with a small 5th wheel in tow, I slipped my TC clutch for miles on end. I could back out of the throttle and watch the tach drop a few hundred RPM then hold. When I stuffed my foot back into it, the tach would raise those 100-200 rpm with no speed increase. This was all done with a mystery switch forcing (or trying to) lockup.
I noticed my transmission temps heating considerably, but this was only after 10-20 minutes of slipping. If I remember correctly, I was backing out of the throttle (reducing slippage) to keep transmission temps at 220° or less. When the TC is locked the transmission temp is more like 140-160°, so there was some definate heat buildup.
I once got stuck on the side of the road and threw it into 4 wheel high instead of 4 wheel low. I stepped on the go pedal and she revved up but the tires never moved. I threw it in 4-low and spun my way out. There was actually white smoke comming from my transmission and the normal horrible burnt clutch smell. I never saw a temp increase during that.
With all of DTT's goodies (including a TC saver), I slipped the front clutch a few times. I have a 1/4 mile near my house and many times (before I got the pressures higher) I would run a quarter, drive straight home for more engine mods, and burnt clutch smell would be radiating from the truck for 5-10 minutes after shutdown. Occasionally, the smell would even enter the cab if I stopped long enough. I never saw a noticable increase in temperatures and could slip the clutch for 1/4 mile at a time.
Even if your transmission is slipping, I doubt you'd see it on the temp gauge if it doesn't occur for several minutes at a time. In case you were wondering, my gauge is in the hot line going to the heat exchanger, not in the pan.
As boost builds in a diesel, the stall speed of the TC increases. That makes it hard to tell by watching the tach whether the transmission is slipping or if the stall is just increasing.
Maybe my experience will help you. If my truck isn't stinking of burnt clutch, I don't worry about it.
-Chris