There is no way to turn off the font-size slider, it is always set to something, so I think all you can do is set it to a known font size and calculate your pixel font sizes relative to the known base size.
For example, if you set the slider to size 50, then a base font size of 200% will corespond to 100 pixels, and so a paragraph font size of 1% should correspond to a of size of 1px.
Code:
<body style="font-size:200%;">
<p style="font-size:10%;">10 pixel font.</p>
<p style="font-size:20%;">20 pixel font.</p>
(On the unpatched firmware, size 50 is position 20 on the 24-position slider. Edit: I'm assuming the slider sizes are in pixels, but I'm not certain of that.)