An early welcome to Virginia is in order. We hope you enjoy your visit.
The reason you found nothing in our statutes that addresses open carry is because there isn't any such thing. Open carry is the normal mode (read that as standard or default) of carry here.
"1. When OC-ing, is a "retention holster" required? (Thumb break, or other "active" retention feature)
From what I've read, there is not any requirement for any type of specific retention feature when carrying OC, but I've received some feedback from two sources that I should use a "retention holster" (their words)... So I thought I would ask, even though I believe the answer is no.
I have a good-quality, leather, molded, OWB holster that I would feel pretty confident jogging in, but it does not have a thumb break or any other specific retention feature, other than snug molding."
As mentioned, there are no requirements for open carry and that includes holsters. However, it is indeed prudent and wise to use a holster with some measure and level of retention when your sidearm is visible. Many of these can be had for very reasonable prices in the kydex variety.
"2. I have a concealed carry license that VA recognizes (TX). If I carry OWB wearing a mix of clothing that at times will not cover the weapon at all and other times might completely cover the weapon until I bend, raise my hands, etc..... Will this cause any problems, practically or legally? To clarify, I might wear a T-shirt tucked in with an OWB holster. Clearly that falls under OC rules. But if I throw on a casual shirt on top of that, a shirt that I already know will do a good job concealing the weapon and holster, until some activity (or maybe a stiff breeze) unconcealed the OWB carry, will this violate any laws or accepted conventions?
What rules do I need to worry about with this type of mostly-covered carry? Is it a no-no or a no-problem?
How is this looked upon/received generally? (I don't want to do anything to harm the OC community by being a poor example.)"
Since OC is the standard carry mode, if your concealed sidearm happens to make itself suddenly visible, that is fine. No worry there. Most people won't notice anyway. And if they do, it is rarely a problem. We do get a lot of visitors and transplants from the northeast and they might get a little wide eyed, But strangely even most of them seem to understand that down here, the carrying of a defensive firearm is a pretty normal thing.
"(In case location matters, I'll be mostly in the Charlottesville/Palmyra area, with some time in Manassas/Battlefield)"
I can't speak for the Charlottesville/Palmyra area but I can for the Manassas area since I live a short distance from Manassas and the battlefield. I carry all the time into Manassas, most always openly, and I have never had even one problem. There are only a few places where businesses have posted no guns signs, though I can't even recall one of those recently. When in doubt, just cover it up. As for the Charlottesville/Palmyra area, send Marco a PM (he might join this discussion anyway). He lives down there and will be most happy to help you out. Great fellow... tell him I mentioned that you contact him.
Once again, welcome early to your visit here in Virginia. When you are in the Manassas area, let me know if you would like to do a lunch. If it's in July, I should be okay with it (have to go in for open back surgery in 5 days). Spend lots of money while you're here.
Oh, BTW. When carrying, be extra pleasant and polite. We like to send positive messages to the rest of the people we encounter so that they see us as responsible and friendly types (somehow, I think you already do that anyway down in Texas).
Hope this helps.