We have not. For AP-level coursework, we have used online college courses at the local CC or regional state uni, but I know this isn't a practical option in many communities, in addition to being sometimes not that affordable (we have state-subsidized dual enrollment). Discussion happened via asynchronous forum postings (or, in some cases, videoconference breakout rooms). With async discussion, the age or nominal grade of posters didn't typically come up, except in one case where students were asked specifically where they were in their education, resulting in two DE students having to out themselves as high school students. Age wasn't an issue at registration, although the institutions in question reserved the right to ask pre-high school age students to come in for an eyeball.
I think the options for online math courses prior to dual enrollment are somewhat better than for humanities, with providers like AOPS carrying students up through calc. The best stable solution for humanities-type discussion I've experienced both as a parent and as a child has been to be part of a multi-sibling/cousin system of GT learners. Which, of course, isn't something most people can control!
