No, there are not any solutions that are both streamable and lossless. This is because the lossy compression is needed to make it streamable (the problem with lossless compression is that it is just too much data to load at real time).
But there are some lossy compressions that are not that lossy visually, and might be perfectly acceptable, even for a transparency mask. You should experiment.
David
Experiment? Like have some hints? What formats does it support? And can you set one video texture as transparency mask for another?
By experiment, I mean try different video formats. At least try one and see what happens. I’m trying to encourage you to discover answers for yourself through experimentation, rather than asking people like us who don’t necessarily know. 
I don’t know the full set of video formats that ffmpeg supports; you’ll have to ask over at the ffmpeg group for that. But there are many variants on video encoding, and ffmpeg seems to support most of them.
But the video format itself isn’t the important part; like JPEG, you can control the amount of lossiness when you encode the video. Virtually every video format supports a bitrate parameter–the higher the bitrate, the more accurate the video (but the larger the file).
You can set one video texture as a transparency mask for another, as I already said above, by using the two-parameter form of loadTexture().
David
Oh, i didnt get what you meant to experiment there.
I didn’t know that panda supports all video formats supported by ffmpeg, I think Ive read a conversation in the forums about some formats not loading.
oops, forgot.