Here is the solution I found.
There might be a better way, but this turns the description on a video embed block into an full overlay caption on hover.
// video embed block caption overlay //
.video-caption-wrapper:hover {
position: absolute;
top: 0px;
bottom: 0px;
left: 0px;
right: 0px;
background: rgba(0,0,0,.5);
padding: 45px 80px 50px 50px;
max-height: 100% !important;
box-sizing: border-box !important;
overflow-y: auto !important;
color: white;
}
.video-caption-wrapper {
position: absolute;
top: 0px;
bottom: 0px;
left: 0px;
right: 0px;
background: rgba(0,0,0,0);
padding: 45px 80px 50px 50px;
max-height: 100% !important;
box-sizing: border-box !important;
overflow-y: auto !important;
color: rgba(0,0,0,0);
}
