OK, this is based on a vague memory of someone asking this before and some quick google research so take it for what it's worth as they say.

The still image option is probably best, especially for longer projects. If you want to output to video then as fivecat says, you can use no compression and get huge files or there are supposedly codecs which support transparency - e.g. reference AVI Export With Alpha Channel

Huffyuv is what I remember someone discussing using with Poser. I think that the Indeo codec may be a part of the standard Windows install.

The HuffyUV 2.1.1 codec is available here 

and the Lagarith codec is here

You're on your own with setting things up in Poser :-)

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken