Well, this seems to be a forgotten corner of the OS in many palms and palm-powered devices.

• Sony - bitdepth 8, the pattern seems to be used, but on the emulator masked the screen with yellow rather than black.
• Handera - bitdepth 2, blanked the screen completely.
• palmOS 3.5 - in bitdepths 8 and 2, wrote alternating bands of grey and white to the screen.
• palmOS 4.0 - worked fine in both bitdepths of 8 and 2.

So, the solution I've arrived at so far works for Handera and PalmOS 4.0+. I'll be improving this to work on some of the other models in time. Here's the code as it sits right now. I *like* long lines, so watch the line wrap...sorry.

void DimScreen()
{
        CustomPatternType pattern = {0xaa,0x55,0xaa,0x55,0xaa,0x55,0xaa,0x55};
        WinHandle winH;
        RectangleType r;
        register unsigned long *d;
        register unsigned char *e;
        register unsigned long s;
        register unsigned short sw;
        register short i,j;
        UInt32 bigPattern[8];
        UInt8 c;
        BitmapType *bm = WinGetBitmap(WinGetDisplayWindow());

        if (pow_drawContext.useHanderaHiRes) {
                if (bm->pixelSize==1) {
                        // overlay the whole screen with a grey pattern.
                        // only works in 1 bit depth for handera.
                        i=0;
                        d=(unsigned long *)(BmpGetBits(bm));
                        while (i<304) {
                                c = pattern[i&7];
                                sw = (short)c<<8 | (short)c;
                                s = (unsigned long)(sw)<<16 | sw;
                                *d++&=~s;*d++&=~s;*d++&=~s;*d++&=~s;*d++&=~s;*d++&=~s;*d++&=~s;
                                e = (unsigned char *)d;
                                *e++ |= (unsigned char)c;
                                *e++ |= (unsigned char)c;
                                d = (unsigned long *)e;
                                i++;
                        }
                } else {
                        d=(unsigned long *)(BmpGetBits(bm));
                        // First, create the bit patterns in bigPattern.
                        // for 2 bit.
                        for (i=0;i<8;i++) {
                                bigPattern[i]=0L;
                                for (j=7;j>-1;j--) {
                                        bigPattern[i] <<= 2;
                                        if (pattern[i] & 1<<j) {
                                                bigPattern[i] |= 3;
                                        }
                                }
                                bigPattern[i] = bigPattern[i]<<16 | bigPattern[i];
                        }
                        // overlay the whole screen with a grey pattern.
                        i=0;
                        while (i<304) {
                                s = bigPattern[i&7];
                                *d++&=~s;*d++&=~s;*d++&=~s;*d++&=~s;*d++&=~s;*d++&=~s;*d++&=~s;
                                *d++&=~s;*d++&=~s;*d++&=~s;*d++&=~s;*d++&=~s;*d++&=~s;*d++&=~s;
                                *d++&=~s;
                                i++;
                        }
                }
        } else if (!pow_drawContext.useSonyHiRes) {
                WinPushDrawState();
            winH = WinSetDrawWindow(WinGetDisplayWindow());
                WinSetPattern((const CustomPatternType *)&pattern);
                WinSetDrawMode(winMask);
                WinGetWindowBounds(&r);
                WinPaintRectangle(&r,0);
            WinSetDrawWindow(winH);
                WinPopDrawState();
        }
}

Hope this has helped. If you have any trouble with this hint, please holler over to Ken through the contact page.