Cold T1 advice

Honestly, I'd just go with your original plan (trisuit on under wetsuit). It really isn't going to take long on this bike before you warm up and my bet is the adrenaline will keep you from feeling cold for very long. Also, drying off in T1 will take forever and trying to get a tight-fitting tri top on when you are even a little wet, fuggetaboutit.

I agree with Oysterboy's advice.

IF you are willing to spend whatever time in T1 using a towel to attempt to dry off enough to get a dry shirt onto a damp body, then perhaps bringing a very absorbent towel into T1 and doing a cursory wipe down might be helpful??

If it is really cold, then a windbreaker cycling jacket with a full zip would be much easier to put on and will help to keep a bit of your body heat trapped.

This all really depends on the individual - some are much more easily affected by the cold.

Have a great race!

Yikes! I'm super cold-blooded and just detest this situation, but it happens even in summer in the Pacific Northwest (where I'm from when not working in the tropics during the school year). My suggestions:

*Unlike a normal race, when you want to take off your cap and goggles as soon as you exit the water, then unzip your wetsuit and get the top half off as you run to your bike, you might want to consider leaving the wetsuit and cap on until you get to your bike, to stay warm a bit longer, especially if transition is big/long. In my last cool morning race (air in mid 40's, water 63), they had wetsuit strippers lakeside and I used them, then realized I was going to be in T1 for almost five minutes due to the size of the event and where my bike was. I did put my neoprene cap back on, but by the time I got on my bike, I was shivering so hard I was fighting to control it for about the first 20 minutes. This also may have led to cramping and poor performance later on the bike leg.

*If weather will continue to be cold throughout the bike leg (i.e. if it's cloudy or windy), you might consider an extra layer. In my experience, the best solution is a fairly snug-fitting pull-on long-sleeved top, especially one with a partial zip (or a full zip top that you can open part-way). Just hang it next to or on the bike in transition. If you get warm, you can pull down the zip or push up the sleeves. I have three such tops--one is actually a surf shirt/rash guard. It fits snugly (but not as tightly as a tri top), goes on well when wet, and provides a little warmth as well as sun protection. Comfortable for temps in the mid 50's to low 60's. I have done a couple of Olys in it. The next "level" is a light merino wool top that is a bit warmer and retains warmth really well when wet. Good for high 40's to mid 50's. Finally, a pull-on windproof bike jersey. I would only race in that if I anticipated temps to not top 50 throughout the bike and windy/rainy conditions. Have actually worn it for a rainy, windy HIM, with no regrets. Key elements for this layer would be snug-fitting but not crazy tight, can pull on when partially unzipped, zipper and sleeves easily adjusted. Don't count on having the dexterity to easily manage a zipper after a cold swim! Arm warmers are likewise a pain in the a$$.

I wouldn't worry too much about drying off--your body heat will do that. Or about legs--they tend to get warm more quickly. It's my experience that what helps most is keeping core warm and head covered through T1 and onto the bike.

Thanks for all the advice. I just went for it and saved some time for sure compared to other. I was 2nd out of the water but was able to pass the first swimmer in T1 and kept first through 19 miles. The only problems I actually had was with my toes and fingers. My toes were frozen! I couldn't feel them until mile 3 of the run.