I'm running a web server using Apache on my home LAN, and recently
obtained some webcam software to stream one of several webcams.  The
software outputs a standard microsoft stream using MMS as the
protocol, on port 8888.

Elsewhere on my LAN, I use Apache's ProxyPass directive in order to
hide other servers, running on different ports, from the outside
world.  My general policy is to close off the port from the Internet
using my firewall, and effectively use ProxyPass and VirtualHost as a
way to obtain restricted access.

However, probably because this time I'm using MMS, I don't seem to be
able do do my usual thing.  Here's my "attempt" in the Apache config
file:

# Secure the web camera 
<VirtualHost *> 
ServerName webcam.mydomain.co.uk 
ProxyPass / http://192.168.0.2:8888/ 
<Location /> 
AllowOverride None 
AuthType Basic 
AuthName "Secured for privacy. Authentication required for access." 
AuthUserFile "c:\directory_to_password_file\.htpasswd" 
Require user myuseraccount 
</Location> 
</VirtualHost> 

I've setup the subdomain "webcam" so all should be working well.  But it isn't.

So, the question is how do I protect an MMS stream with a password? 
Ideally I want to use Apache as the mechanism to throw up a password
request, but am I hoping for the impossible?

you can create a new domain within the webserver and have the stream
exist within that domain.  thus when trying to access the stream you
will be requred to authenticate, just as f it were a folder that
existed within a restricted domain.