The eTrex is a great unit for what you're doing. I was pretty disappointed to find my Oregon won't let me grab real time data, but I wouldn't be surprised at all to learn your eTrex would. My old eMap let me do that.

I'm going to play with the concept. I like the idea of being able to share the GPS with other hikers in a group, and I know it's possible. I'm sure I could probably estimate the power consumption with the specs on the hardware I am considering using, but from what I think I know, I think it's doable.