I tried for a long time to get this to work, and I could get it to work in FS-UAE but not in WinUAE.
http://eab.abime.net/showthread.php?t=89811
It is easy to test by looking at the text - in WinUAE, some pixels are more distorted then others after scaling.
On FS-UAE, my shader "sharp-bilinear-simple" works perfectly and has been included in the Git repo already.