I'm not sure if this works, but it should:

div.pmsig-85580:before {
    display: block;
    text-align: right;
    color: #444;
    content: 'Signature hidden';
}
div.pmsig-85580 {
    display: none;
}
[download]

Update: turns out it needs more tricks. The :before block is considered to be a child of the element it is attached to, so display:block, visibility:hidden or anything else applied to that element will also affect the :before block. So it requires a little cheating:

div.pmsig-85580 {
    height: 1.5em;
    overflow: hidden;
}
div.pmsig-85580:before {
    content: 'Signature hidden';
    color: #444;
    text-align: right;
    height: 1.5em;
    display: block;
}
[download]

Here the box is set to a fixed 1.5em height, all of which are then occupied by the :before block. The rest of the content would normally spill onto the elements following on the page, but is prevented from that by way of overflow:hidden — voilà.

Makeshifts last the longest.