Another (undocumented) approach is to add I()
around your inline code to print the characters as is without the <code>
tag, e.g.
<html>
<head>
<meta name="author" content="<!--rinline I(Sys.getenv('USERNAME')) -->">
<meta name="date" content="<!--rinline I(as.character(Sys.time())) -->">
</head>
<body>
</body>
</html>