is there a way to force a full https in the main website

HI,
is there a way to force a full https in the main website in WPMU. because i don't have a wild card ssl , just have the standard certificate and with standard ssl it will only work on a single site and subdirectory site but won't work on subdomain site. so i used this

RewriteCond %{SERVER_PORT} !^443$
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]

but the problem with it , it forces https accross all the subsites , i just want to force the https to the main site.

THanks in advanced for help / advice.