Apache2 RewriteRule syntax for ActiveSync redirect in reverse proxy

Apache2 RewriteRule syntax for ActiveSync redirect in reverse proxy

Okay, I think this is a fairly simple task, but I'm having a little
trouble testing reliably and working out what I've done wrong. I tried to
follow the Apache mod_rewrite documentation, but it doesn't appear to be
working the way I expected.
I have an Apache server running as a reverse proxy in front of an Exchange
CAS for public OWA access, and we want to intercept ActiveSync traffic
(simple pattern match) and redirect it to an AirWatch Secure Email Gateway
(SEG) URL. All other (i.e. web browser OWA) traffic should be sent along
to the CAS as usual.
So what we started with is something like this, and this works perfectly
for OWA access, but doesn't do the ActiveSync-to-SEG redirect:
<VirtualHost *:443>
ServerName webmail.company.com:443
RewriteEngine on
RewriteRule ^/$ https://webmail.company.com/exchange [R,L]
<Location />
Order allow,deny
Allow from all
# This is the CAS's internal IP
ProxyPass https://10.100.10.209/
ProxyPassReverse https://10.100.10.209/
</Location>
</VirtualHost>
So I added this line (the one in the middle) in an attempt to rewrite URLs
that begin with "/Microsoft-Server-Activesync/[whatever]" into
"https://seg.company.com/Microsoft-Server-Activesync/[whatever]":
<VirtualHost *:443>
...
RewriteEngine on
RewriteRule ^/Microsoft-Server-Activesync?(.+)$
https://seg.company.com/Microsoft-Server-Activesync?$1 [R]
RewriteRule ^/$ https://webmail.company.com/exchange [R,L]
...
</VirtualHost>
When I made this change (in test, of course), the traffic I'm sending to
it from a mobile device does not appear to be getting redirected. Instead
the ActiveSync traffic just flows along to the Exchange CAS as usual like
the rule is not being triggered.
So, two questions:
Am I doing something obvious wrong with the RewriteRule I added?, and
How can I troubleshoot this a little more intelligently than, "Hit
ActiveSync URL in a browser, check web server logs to find out where it
went"?
Thanks in advance!
Regards, Jon Heese