That does leave one problem: you still need a way to segment your stream. Most length-prefixed framing schemes do not have any way to segment the stream other than the length prefix. What you wind up wanting is something like chunked encoding.
(Also, using zero as a sentinel is not necessarily a good idea, since it makes zero length messages more difficult. I'd go with -1 or ~0 instead.)
Using `-1` would require using a singed integer for the length, which I guess could be done if you're fine with having the maximum length be half as long, but that also raises the question of what to do with the remaining negative values; what does a length of -10 mean?
I thought -0 is only something in floating point numbers, not integers, and using floats for the length of a message sounds like a nightmare to me.
Ah, I was a little unclear. I mean ~0 as in NOT 0, an integer with all bits set. This is also the same as -1 in two's compliment. So basically, I'm suggesting you use the maximum unsigned integer value as a sentinel. That doesn't work if you're using a variable-length unsigned integer like base128vlq, but if you're doing base128vlq you could always make a special sentinel for that (e.g. unnecessarily set the high bit and follow it with a zero byte; this would never normally appear with a base128vlq.)
(Also, using zero as a sentinel is not necessarily a good idea, since it makes zero length messages more difficult. I'd go with -1 or ~0 instead.)