Doesn't apply to wp-admin

Thanks for the rating. Why would you want to compress the admin HTML, though? It isn’t accessed nearly as much as the public content and can’t be cached, so it’d just slow down the server.

I’m interested more on the absolute to relative URL link conversion than compression. I need this feature because I want to offer wp-admin part from a different host, for example https://admin.sample.com so that this admin host can be secured by enterprise SiteMinder SSO. This leaves public facing site https://sample.com untouched by SiteMinder, which is not a highly stable product. Providing a separate host for wp-admin can be easily achieved through Apache mod_rewrite and r-proxy, but without relative URL links, every subsequent request from wp-admin results in a client-side redirect, and hence more performance penalty to site admins.
It would be even better if you can offer an option to enable/disable URL conversion and HTML compression separately.

I’ll keep it in mind for a future version when an admin panel is made available.

In the meantime, if you upgrade to v0.5.4.2, open up wp-html-compression.php and comment out lines 35, 36, 41–46. That will enable the plugin for the admin, but it will also minify the HTML, thereby slowing down each page load by a small fraction of a second.