I think the only way to cover all your bases is to stick a position: fixed footer on your forum that is always visible, unlike the default footer that only becomes visible after scrolling all the way down through several content updates.
For a quick test, I put this in the </body> section of my customization:
<div class="imprint">put links to more legal cruft here</div>
And this is the corresponding CSS with some basic but ugly styling that somewhat blends in with my overall customization:
.imprint {
position: fixed;
bottom: 0;
left: 0;
right: 0;
width: 1140px;
margin: 0 auto;
height: 20px;
background-color: #eee;
color: #666;
text-align: center;
border-top: 1px solid #888;
z-index: 100;
}
And if you want it to cover the entire viewport width, get rid of left, right and margin and set width to 100%.