bobbau, couldn't you do it in single-pass multitexturing by doing something like this:
pd3dDevice->SetTextureStageState( 0, D3DTSS_COLOROP, D3DTOP_SELECTARG1 );
pd3dDevice->SetTextureStageState( 0, D3DTSS_COLORARG1, D3DTA_TEXTURE );
pd3dDevice->SetTextureStageState( 0, D3DTSS_ALPHAOP, D3DTOP_SELECTARG1 );
pd3dDevice->SetTextureStageState( 0, D3DTSS_ALPHAARG1, D3DTA_TEXTURE );
pd3dDevice->SetTextureStageState( 1, D3DTSS_COLOROP, D3DTOP_ADD );
pd3dDevice->SetTextureStageState( 1, D3DTSS_COLORARG1, D3DTA_TEXTURE );
pd3dDevice->SetTextureStageState( 1, D3DTSS_COLORARG2, D3DTA_CURRENT );
pd3dDevice->SetTextureStageState( 1, D3DTSS_ALPHAOP, D3DTOP_DISABLE );
Oh, wait, then the glow map wouldn't be fullbright. never mind then.