Until a non-technical/friendly solution comes along ...
Sounds like the quickest thing to do at the moment, I think, is to have a dinky "loader" html page that loads a TiddlyWiki in an iframe, keeping the iframe hidden until TiddlyWiki is fully loaded, and showing splash in the meantime.
I imagine that needs some javascript, and me not being a fan, Ieave that to somebody else.
<html>
<head>
<title> Le P'tit Aurèle, un lexique acadien</title>
<style>
body, html {width: 100%; height: 100%; margin: 0; padding: 0}
.first-row {position: absolute;top: 0; left: 0; right: 0; height: 100px; background-color: lime;}
.second-row {position: absolute; top: 100px; left: 0; right: 0; bottom: 0; background-color: red }
.second-row iframe {display: block; width: 100%; height: 100%; border: none;}
</style>
</head>
<body>
<div class="first-row">
<p>The TiddlyWiki is loading</p>
</div>
<div class="second-row">
</div>
</body>