Here's my experience if you're still interested.
We towed a 26' aluminum
boat behind the 100' yacht that I ran. Much of the time we towed at 17 knots and if we weren't, we were cruising at 12 knots. We used a 1 1/4" synthetic tow line. You will want something with stretch. Depending on the direction (relative to our direction of travel) and frequency of the waves, there were days I could run 17 knots in 4'-5' seas with no issues.
1) types of harness/bridles
Obviously, there are issues with towing at speed and the drag of the vessel being towed. The largest issue is going to be the strength of the eye and the material (presumably fiberglass) surrounding it. My advice would be to try to strengthen the eye if possible and disperse the load. If you have the ability to hide the backing (depending on your model, this may not work), I would suggest having a shop make you an aluminum
plate that the eye screws through.
If you have stronger breast cleats than a tow eye, I would recommend making a bridle that leads to the eye (in case the eye comes out) or going straight to the cleats. I don't feel that the hardware
on many of the cleats allows for much pulling especially in a shearing direction, so it wouldn't be my first choice.
2) tow boat motor up/down
I noticed someone suggested pulling the motor off. I don't think this is smart or practical for your size tender. I would leave the motor on to help keep the weight in the back of the boat and the bow up. However, you should pull it up and lock it in the 'up' position. In lumpy seas the motor will bounce around and you don't want the ram taking all of the shock if you can help it.
3) effective length of bridle.
This can vary from boat to boat both on the towing and towed ends. I would say the absolute least amount of line would be 100'. I think I would plan for between 150'-200'. Regardless of how much you are actually using most of the time, you may want to keep the rest around in case you get into rough seas and need to add some line. I would say you should fashion a quick coupling of some sort so that you can insert it quickly and easily.
4) how hairy does it get in rough seas
It can get very hairy in rough seas and it means that you: a) try not to travel in rough seas and b) drive your main vessel by paying close attention to both boats and ensuring that you aren't towing your tender through waves or jerking on it too hard.
One way to accomplish this is to add weight (a large downrigger ball or something like it) to the middle of your tow line. This provides a shock absorber in between the two boats. The issue in rough seas is that the boats aren't moving in unison and the larger boat wins. When it does, it makes the tender do things (usually bad things) that it doesn't want to do.
7) how much impact on fuel burn?
I wouldn't worry about your fuel burn too much. At those speeds, you aren't winning any arguments with electric
car owners. The tender will be on plane and I wouldn't think that you would notice much more than a 10 gallon difference on a weekend. Clearly, all boats are different and you would have to do extensive testing, but I doubt you will notice unless you really pay attention.
I have had a tow eye rip out of the aluminum tender when towing. It's a pretty easy fix because aluminum isn't overly pretty to start with. I just thought it was worth pointing out that tow eyes really aren't made for huge amounts of strain. If you can, you should reinforce it.
I also recommend that you continue to keep an eye on the boat. If it starts doing something it hasn't before, start asking why. Try to learn it's habits as you've done with your Silverton so that you know when something doesn't seem right.
I hope that this has helped clear some things up for you. There are some items that I didn't address that you've probably thought of. I would be happy to continue this discussion either online or off if you would like to go over it some more.
Good luck and safe boating