It's a bit academic for me, as the dress code specifies trousers for guys even on casual Fridays, although probably just because whoever wrote it couldn't imagine a guy wearing anything else.

Not only that, but I always wear dark briefs, whether kilted or not. FWIW, it seems to me that anything that extends part way down the leg is MORE likely to show under a kilt than briefs are. On top of that, there is also the possibility of an unwanted view up the leg of boxers (oh, the horror), and that can't happen with briefs.

The chance of a kilt blowing up in the wind is the least likely way that anyone will see up your kilt. IME the risk of that is very low, even with the lightest weight kilt. Especially indoors, LOL! It may happen when hill walking or standing on the seafront, but unless those kind of things are in your job description there is no real need to worry about it.

The greatest risk of exposure is when simply sitting down, and even if your desk has a modesty panel for the benefit of the ladies you may well find yourself sitting in other places, e.g. when in someone else's office, or when eating lunch. Climbing stairs is a very high risk activity, but usually you have the option of taking the lift/elevator instead. Of course, if your job requires you to crawl on the floor, then a kilt may not be a good idea, as even assuming the floors are clean it is hard to keep covered up when doing that.

If for some reason I ever wore a kilt to the office, I wouldn't dream of going regimental, but then I wouldn't want to anyway. Nothing in my job involves anything more strenuous than sitting down or walking around, and the stairs only get used if there's a fire drill, so I can't imagine wearing a kilt would be in any way problematic, apart from those pesky rules.