Microsoft DHCP server with option 252 (proxy pac)

That is correct.  The only other suggestion I can provide if you go wtih the GPO option is to place a shortcut on the user's desktop to disable the proxy setting via a change in the registry. 

Enable and Disable Proxy Settings via Shortcut
http://www.anitkb.com/2011/07/enable-and-disable-proxy-settings.html

I havent really used option 252 in DHCP for configuring proxy settings.  however, here are some articles that may be of use:

"Automatically Detect Settings" Does Not Work if You Configure DHCP Option 252
http://support.microsoft.com/kb/307502

Automatic Discovery for Firewall and Web Proxy Clients
http://technet.microsoft.com/en-us/library/cc713344.aspx

Create an option 252 entry in DHCP
http://technet.microsoft.com/en-us/library/bb794881.aspx