[jQuery] blockUI: allowBodyStretch not working

[jQuery] blockUI: allowBodyStretch not working

Unfortunately, the wonderful blockUI plugin doesn't grey out the full
background on short pages. I tried the
$.blockUI.defaults.allowBodyStretch = true; but that doesn't help
FireFox 3.5.7 on Ubuntu 9.10
.
Digging a bit deeper with Firebug, I noticed that the grey overlay
carries a property of position: absolute while other tools that grey
out the background
like. e.g., UserVoice
use position: fixed.
Consequently, I tried $.blockUI.defaults.overlayCSS.position = 'fixed'
but that doesn't find its way down to the style used.
Can anyone help?